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[elpa] externals/compat 10033e0 01/99: Initial import


From: ELPA Syncer
Subject: [elpa] externals/compat 10033e0 01/99: Initial import
Date: Sun, 17 Oct 2021 05:57:46 -0400 (EDT)

branch: externals/compat
commit 10033e07a89e3405ee27797e67cb1c4521e4f5a2
Author: Philip Kaludercic <philipk@posteo.net>
Commit: Philip Kaludercic <philipk@posteo.net>

    Initial import
---
 .build.yml      |   10 +
 .dir-locals.el  |    4 +
 .elpaignore     |    3 +
 .gitignore      |    5 +
 COPYING         |  674 +++++++++++++++++++++++++++++++
 Makefile        |   28 ++
 README          |   62 +++
 compat-24.2.el  |   60 +++
 compat-24.3.el  |   44 ++
 compat-24.4.el  |  142 +++++++
 compat-25.1.el  |  239 +++++++++++
 compat-26.1.el  |  286 +++++++++++++
 compat-27.1.el  |  271 +++++++++++++
 compat-28.1.el  |  456 +++++++++++++++++++++
 compat-macs.el  |  245 ++++++++++++
 compat-tests.el | 1195 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 compat.el       |   90 +++++
 17 files changed, 3814 insertions(+)

diff --git a/.build.yml b/.build.yml
new file mode 100644
index 0000000..3d6d760
--- /dev/null
+++ b/.build.yml
@@ -0,0 +1,10 @@
+image: alpine/edge
+packages:
+- emacs
+tasks:
+- setup: |
+    apk add emacs
+- build: |
+    make -C compat compile
+- check: |
+    make -C compat test
\ No newline at end of file
diff --git a/.dir-locals.el b/.dir-locals.el
new file mode 100644
index 0000000..1f7c4b7
--- /dev/null
+++ b/.dir-locals.el
@@ -0,0 +1,4 @@
+;;; Directory Local Variables
+;;; For more information see (info "(emacs) Directory Variables")
+
+((emacs-lisp-mode . ((indent-tabs-mode . nil))))
diff --git a/.elpaignore b/.elpaignore
new file mode 100644
index 0000000..8969c4b
--- /dev/null
+++ b/.elpaignore
@@ -0,0 +1,3 @@
+compat-tests.el
+Makefile
+.build.yml
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..a3d210a
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,5 @@
+*.elc
+*~
+\#*\#
+/compat-pkg.el
+/compat-autoloads.el
\ No newline at end of file
diff --git a/COPYING b/COPYING
new file mode 100644
index 0000000..ba8389a
--- /dev/null
+++ b/COPYING
@@ -0,0 +1,674 @@
+                   GNU GENERAL PUBLIC LICENSE
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+any implied license or other defenses to infringement that may
+otherwise be available to you under applicable patent law.
+
+  12. No Surrender of Others' Freedom.
+
+  If conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot convey a
+covered work so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you may
+not convey it at all.  For example, if you agree to terms that obligate you
+to collect a royalty for further conveying from those to whom you convey
+the Program, the only way you could satisfy both those terms and this
+License would be to refrain entirely from conveying the Program.
+
+  13. Use with the GNU Affero General Public License.
+
+  Notwithstanding any other provision of this License, you have
+permission to link or combine any covered work with a work licensed
+under version 3 of the GNU Affero General Public License into a single
+combined work, and to convey the resulting work.  The terms of this
+License will continue to apply to the part which is the covered work,
+but the special requirements of the GNU Affero General Public License,
+section 13, concerning interaction through a network will apply to the
+combination as such.
+
+  14. Revised Versions of this License.
+
+  The Free Software Foundation may publish revised and/or new versions of
+the GNU General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+  Each version is given a distinguishing version number.  If the
+Program specifies that a certain numbered version of the GNU General
+Public License "or any later version" applies to it, you have the
+option of following the terms and conditions either of that numbered
+version or of any later version published by the Free Software
+Foundation.  If the Program does not specify a version number of the
+GNU General Public License, you may choose any version ever published
+by the Free Software Foundation.
+
+  If the Program specifies that a proxy can decide which future
+versions of the GNU General Public License can be used, that proxy's
+public statement of acceptance of a version permanently authorizes you
+to choose that version for the Program.
+
+  Later license versions may give you additional or different
+permissions.  However, no additional obligations are imposed on any
+author or copyright holder as a result of your choosing to follow a
+later version.
+
+  15. Disclaimer of Warranty.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                    END OF TERMS AND CONDITIONS
+
+           How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..e218aa6
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,28 @@
+.POSIX:
+.PHONY: all compile test clean
+.SUFFIXES: .el .elc
+
+EMACS = emacs
+BYTEC = compat-24.2.elc \
+       compat-24.3.elc \
+       compat-24.4.elc \
+       compat-25.1.elc \
+       compat-26.1.elc \
+       compat-27.1.elc \
+       compat-28.1.elc \
+       compat-macs.elc \
+       compat.elc
+
+all: compile test
+
+compile: $(BYTEC)
+
+test:
+       $(EMACS) -Q --batch -L . -l compat-tests.el -f 
ert-run-tests-batch-and-exit
+
+clean:
+       rm -f $(BYTEC)
+
+.el.elc:
+       $(EMACS) -Q --batch -L . -f batch-byte-compile $^
+
diff --git a/README b/README
new file mode 100644
index 0000000..0a40bae
--- /dev/null
+++ b/README
@@ -0,0 +1,62 @@
+* COMPATibility Library for Emacs               -*- mode: outline; -*-
+
+Find here the source for compat.el, a forwards-compatibility library
+for (GNU) Emacs Lisp.
+
+The intended audience of this library aren't day-to-day users, but
+package developers that wish to make use of newer functionality, not
+provided in older versions of Emacs, without breaking compatibility
+for users bound to specific Emacs releases.
+
+** Installation
+
+compat.el shouldn't be installed directly, but downloaded from GNU
+ELPA.  If necessary, this can be done using M-x package-install-file
+in the current working directory.
+
+** Usage
+
+The intended use-case for this library is for package developers to
+add as a dependency in the header:
+
+    ;; Package-Requires: ((emacs "24.3") (compat "28.1.0.0"))
+
+No further action should be required afterwards.  The effect should be
+that all the functions and macros that compat.el provides are
+automatically accessible or made accessible as soon as the right
+libraries are loaded.
+
+** Contribute
+
+As compat.el is distribed as part of GNU ELPA, and therefore requires
+a copyright assignment to the FSF, for all non-trivial code
+contributions:
+
+    
https://www.gnu.org/software/emacs/manual/html_node/emacs/Copyright-Assignment.html
+
+*** Source code
+
+The project is managed can be found on SourceHut
+
+    https://sr.ht/~pkal/compat
+
+but has a (disfavoured) GitHub mirror as well
+
+    https://github.com/phikal/compat.el
+
+*** Bug and patches
+
+Patches, bug reports and comments can be sent to the mailing list
+
+    ~pkal/public-inbox@lists.sr.ht
+
+When contributing, make sure to provide test and use the existing
+tests defined in compat-test.el.  These can be easily executed using
+the bundled Makefile:
+
+    make test
+
+** Distribution
+
+compat.el and all other files in this directory are distributed under
+the GPL, Version 3 (like Emacs itself).
diff --git a/compat-24.2.el b/compat-24.2.el
new file mode 100644
index 0000000..bc1a15d
--- /dev/null
+++ b/compat-24.2.el
@@ -0,0 +1,60 @@
+;;; compat-24.2.el --- Compatibility Layer for Emacs 24.2  -*- 
lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Find here the functionality added in Emacs 24.2, needed by older
+;; versions.
+;;
+;; Do NOT load this library manually.  Instead require `compat'.
+
+;;; Code:
+
+(eval-and-compile (require 'compat-macs))
+
+;;;; Defined in subr.el
+
+(compat-defun autoloadp (object)
+  "Non-nil if OBJECT is an autoload."
+  (eq 'autoload (car-safe object)))
+
+(compat-defun buffer-narrowed-p ()
+  "Return non-nil if the current buffer is narrowed."
+  (/= (- (point-max) (point-min)) (buffer-size)))
+
+(compat-defun posnp (obj)
+  "Return non-nil if OBJ appears to be a valid position.
+A `posn' object is returned from functions such as `event-start'.
+If OBJ is a valid `posn' object, but specifies a frame rather
+than a window, return nil."
+  (and (windowp (car-safe obj))
+       (atom (car-safe (setq obj (cdr obj))))                ;AREA-OR-POS.
+       (integerp (car-safe (car-safe (setq obj (cdr obj))))) ;XOFFSET.
+       (integerp (car-safe (cdr obj)))))
+
+;;;; Defined in files.el
+
+(compat-defun file-name-base (filename)
+  "Return the base name of the FILENAME: no directory, no extension."
+  (file-name-sans-extension
+   (file-name-nondirectory (or filename (buffer-file-name)))))
+
+(provide 'compat-24.2)
+;;; compat-24.2.el ends here
diff --git a/compat-24.3.el b/compat-24.3.el
new file mode 100644
index 0000000..93a7be3
--- /dev/null
+++ b/compat-24.3.el
@@ -0,0 +1,44 @@
+;;; compat-24.3.el --- Compatibility Layer for Emacs 24.3  -*- 
lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Find here the functionality added in Emacs 24.3, needed by older
+;; versions.
+;;
+;; Do NOT load this library manually.  Instead require `compat'.
+
+;;; Code:
+
+(eval-and-compile (require 'compat-macs))
+
+;;;; Defined in subr.el
+
+(compat-defmacro defvar-local (var val &optional docstring)
+  "Define VAR as a buffer-local variable with default value VAL.
+Like `defvar' but additionally marks the variable as being automatically
+buffer-local wherever it is set."
+  (declare (debug defvar) (doc-string 3))
+  `(progn
+     (defvar ,var ,val ,docstring)
+     (make-variable-buffer-local ',var)))
+
+(provide 'compat-24.3)
+;;; compat-24.3.el ends here
diff --git a/compat-24.4.el b/compat-24.4.el
new file mode 100644
index 0000000..b744562
--- /dev/null
+++ b/compat-24.4.el
@@ -0,0 +1,142 @@
+;;; compat-24.4.el --- Compatibility Layer for Emacs 24.4  -*- 
lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Find here the functionality added in Emacs 24.4, needed by older
+;; versions.
+;;
+;; Do NOT load this library manually.  Instead require `compat'.
+
+;;; Code:
+
+(eval-and-compile (require 'compat-macs))
+
+;;;; Defined in data.c
+
+(compat-advise = (number-or-marker &rest numbers-or-markers)
+  "Handle multiple arguments."
+  (catch 'fail
+    (while numbers-or-markers
+      (unless (funcall oldfun number-or-marker (car numbers-or-markers))
+        (throw 'fail nil))
+      (setq number-or-marker (pop numbers-or-markers)))
+    t))
+
+(compat-advise < (number-or-marker &rest numbers-or-markers)
+  "Handle multiple arguments."
+  (catch 'fail
+    (while numbers-or-markers
+      (unless (funcall oldfun number-or-marker (car numbers-or-markers))
+        (throw 'fail nil))
+      (setq number-or-marker (pop numbers-or-markers)))
+    t))
+
+(compat-advise > (number-or-marker &rest numbers-or-markers)
+  "Handle multiple arguments."
+  (catch 'fail
+    (while numbers-or-markers
+      (unless (funcall oldfun number-or-marker (car numbers-or-markers))
+        (throw 'fail nil))
+      (setq number-or-marker (pop numbers-or-markers)))
+    t))
+
+(compat-advise <= (number-or-marker &rest numbers-or-markers)
+  "Handle multiple arguments."
+  (catch 'fail
+    (while numbers-or-markers
+      (unless (funcall oldfun number-or-marker (car numbers-or-markers))
+        (throw 'fail nil))
+      (setq number-or-marker (pop numbers-or-markers)))
+    t))
+
+(compat-advise >= (number-or-marker &rest numbers-or-markers)
+  "Handle multiple arguments."
+  (catch 'fail
+    (while numbers-or-markers
+      (unless (funcall oldfun number-or-marker (pop numbers-or-markers))
+        (throw 'fail nil)))
+    t))
+
+;;;; Defined in subr.el
+
+(compat-defmacro with-eval-after-load (file &rest body)
+  "Execute BODY after FILE is loaded.
+FILE is normally a feature name, but it can also be a file name,
+in case that file does not provide any feature.  See `eval-after-load'
+for more details about the different forms of FILE and their semantics."
+  (declare (indent 1) (debug (form def-body)))
+  `(eval-after-load ,file (lambda () ,@body)))
+
+(compat-defun special-form-p (object)
+  "Non-nil if and only if OBJECT is a special form."
+  (if (and (symbolp object) (fboundp object))
+      (setq object (indirect-function object)))
+  (and (subrp object) (eq (cdr (subr-arity object)) 'unevalled)))
+
+(compat-defun macrop (object)
+  "Non-nil if and only if OBJECT is a macro."
+  (let ((def (indirect-function object)))
+    (when (consp def)
+      (or (eq 'macro (car def))
+          (and (autoloadp def) (memq (nth 4 def) '(macro t)))))))
+
+(compat-defun string-suffix-p (suffix string  &optional ignore-case)
+  "Return non-nil if SUFFIX is a suffix of STRING.
+If IGNORE-CASE is non-nil, the comparison is done without paying
+attention to case differences."
+  (let ((start-pos (- (length string) (length suffix))))
+    (and (>= start-pos 0)
+         (eq t (compare-strings suffix nil nil
+                                string start-pos nil ignore-case)))))
+
+(compat-advise split-string (string &optional separators omit-nulls trim)
+  "Handle optional argument TRIM."
+  (let* ((token (funcall oldfun string separators omit-nulls))
+         (trimmed (if trim
+                      (mapcar
+                       (lambda (token)
+                         (when (string-match (concat "\\`" trim) token)
+                           (setq token (substring token (match-end 0))))
+                         (when (string-match (concat trim "\\'") token)
+                           (setq token (substring token 0 (match-beginning 
0))))
+                         token)
+                       token)
+                    token)))
+    (if omit-nulls (delete "" trimmed) trimmed)))
+
+(compat-defun delete-consecutive-dups (list &optional circular)
+  "Destructively remove `equal' consecutive duplicates from LIST.
+First and last elements are considered consecutive if CIRCULAR is
+non-nil."
+  (let ((tail list) last)
+    (while (cdr tail)
+      (if (equal (car tail) (cadr tail))
+          (setcdr tail (cddr tail))
+        (setq last tail
+              tail (cdr tail))))
+    (if (and circular
+             last
+             (equal (car tail) (car list)))
+        (setcdr last nil)))
+  list)
+
+(provide 'compat-24.4)
+;;; compat-24.4.el ends here
diff --git a/compat-25.1.el b/compat-25.1.el
new file mode 100644
index 0000000..a8010e0
--- /dev/null
+++ b/compat-25.1.el
@@ -0,0 +1,239 @@
+;;; compat-25.1.el --- Compatibility Layer for Emacs 25.1  -*- 
lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Find here the functionality added in Emacs 25.1, needed by older
+;; versions.
+;;
+;; Do NOT load this library manually.  Instead require `compat'.
+
+;;; Code:
+
+(eval-and-compile (require 'compat-macs))
+
+;;;; Defined in fns.c
+
+(compat-advise sort (seq predicate)
+  "Handle SEQ of type vector."
+  (cond
+   ((listp seq)
+    (funcall oldfun seq predicate))
+   ((vectorp seq)
+    (let ((cseq (sort (append seq nil) predicate)))
+      (dotimes (i (length cseq))
+        (setf (aref seq i) (nth i cseq)))
+      (apply #'vector cseq)))
+   ((signal 'wrong-type-argument 'list-or-vector-p))))
+
+;;;; Defined in editfns.c
+
+(compat-defun format-message (string &rest objects)
+  "Format a string out of a format-string and arguments.
+The first argument is a format control string.
+The other arguments are substituted into it to make the result, a string.
+
+This implementation is equivalent to `format'."
+  (apply #'format string objects))
+
+;;;; Defined in minibuf.c
+
+;; TODO advise read-buffer to handle 4th argument
+
+;;;; Defined in subr.el
+
+(compat-defun string-greaterp (string1 string2)
+  "Return non-nil if STRING1 is greater than STRING2 in lexicographic order.
+Case is significant.
+Symbols are also allowed; their print names are used instead."
+  (string-lessp string2 string1))
+
+(compat-defmacro with-file-modes (modes &rest body)
+  "Execute BODY with default file permissions temporarily set to MODES.
+MODES is as for `set-default-file-modes'."
+  (declare (indent 1) (debug t))
+  (let ((umask (make-symbol "umask")))
+    `(let ((,umask (default-file-modes)))
+       (unwind-protect
+           (progn
+             (set-default-file-modes ,modes)
+             ,@body)
+         (set-default-file-modes ,umask)))))
+
+;;;; Defined in subr-x.el
+
+(compat-defmacro if-let* (varlist then &rest else)
+  "Bind variables according to VARLIST and evaluate THEN or ELSE.
+This is like `if-let' but doesn't handle a VARLIST of the form
+\(SYMBOL SOMETHING) specially."
+  :feature subr-x
+  (declare (indent 2)
+           (debug ((&rest [&or symbolp (symbolp form) (form)])
+                   body)))
+  `(let* (,(let ((empty (make-symbol "s"))
+                 (last t) list)
+             (dolist (var varlist)
+               (push `(,(if (cdr var) (car var) empty)
+                       (and ,last ,(or (cdr var) (car var))))
+                     list)
+               (when (or (cdr var) (consp (car var)))
+                 (setq last (caar list))))
+             (nreverse list)))
+     ,then ,@else))
+
+(compat-defmacro when-let* (varlist &rest body)
+  "Bind variables according to VARLIST and conditionally evaluate BODY.
+This is like `when-let' but doesn't handle a VARLIST of the form
+\(SYMBOL SOMETHING) specially."
+  :feature subr-x
+  (declare (indent 1) (debug compat--if-let*))
+  `(compat--if-let* ,varlist ,(macroexp-progn body)))
+
+(compat-defmacro and-let* (varlist &rest body)
+  "Bind variables according to VARLIST and conditionally evaluate BODY.
+Like `when-let*', except if BODY is empty and all the bindings
+are non-nil, then the result is non-nil."
+  :feature subr-x
+  (declare (indent 1) (debug compat--if-let*))
+  (let ((empty (make-symbol "s"))
+        (last t) list)
+    (dolist (var varlist)
+      (push `(,(if (cdr var) (car var) empty)
+              (and ,last ,(or (cdr var) (car var))))
+            list)
+      (when (or (cdr var) (consp (car var)))
+        (setq last (caar list))))
+    `(let* (,(nreverse list))
+       (when ,(caar list)
+         ,@body))))
+
+(compat-defmacro if-let (spec then &rest else)
+  "Bind variables according to SPEC and evaluate THEN or ELSE.
+Evaluate each binding in turn, as in `let*', stopping if a
+binding value is nil.  If all are non-nil return the value of
+THEN, otherwise the last form in ELSE.
+
+Each element of SPEC is a list (SYMBOL VALUEFORM) that binds
+SYMBOL to the value of VALUEFORM.  An element can additionally be
+of the form (VALUEFORM), which is evaluated and checked for nil;
+i.e. SYMBOL can be omitted if only the test result is of
+interest.  It can also be of the form SYMBOL, then the binding of
+SYMBOL is checked for nil.
+
+As a special case, interprets a SPEC of the form \(SYMBOL SOMETHING)
+like \((SYMBOL SOMETHING)).  This exists for backward compatibility
+with an old syntax that accepted only one binding."
+  :feature subr-x
+  (declare (indent 2)
+           (debug ([&or (symbolp form)
+                        (&rest [&or symbolp (symbolp form) (form)])]
+                   body)))
+  (when (and (<= (length spec) 2)
+             (not (listp (car spec))))
+    ;; Adjust the single binding case
+    (setq spec (list spec)))
+  `(compat--if-let* ,spec ,then ,@(macroexp-unprogn else)))
+
+(compat-defmacro when-let (spec &rest body)
+  "Bind variables according to SPEC and conditionally evaluate BODY.
+Evaluate each binding in turn, stopping if a binding value is nil.
+If all are non-nil, return the value of the last form in BODY.
+
+The variable list SPEC is the same as in `if-let'."
+  :feature subr-x
+  (declare (indent 1) (debug compat--if-let))
+  (list 'compat--if-let spec (macroexp-progn body)))
+
+(compat-defmacro thread-first (&rest forms)
+  "Thread FORMS elements as the first argument of their successor.
+Example:
+    (thread-first
+      5
+      (+ 20)
+      (/ 25)
+      -
+      (+ 40))
+Is equivalent to:
+    (+ (- (/ (+ 5 20) 25)) 40)
+Note how the single `-' got converted into a list before
+threading."
+  :feature subr-x
+  (declare (indent 1)
+           (debug (form &rest [&or symbolp (sexp &rest form)])))
+  (let ((body (car forms)))
+    (dolist (form (cdr forms))
+      (when (symbolp form)
+        (setq form (list form)))
+      (setq body (append (list (car form))
+                         (list body)
+                         (cdr form))))
+    body))
+
+(compat-defmacro thread-last (&rest forms)
+  "Thread FORMS elements as the last argument of their successor.
+Example:
+    (thread-last
+      5
+      (+ 20)
+      (/ 25)
+      -
+      (+ 40))
+Is equivalent to:
+    (+ 40 (- (/ 25 (+ 20 5))))
+Note how the single `-' got converted into a list before
+threading."
+  :feature subr-x
+  (declare (indent 1) (debug thread-first))
+  (let ((body (car forms)))
+    (dolist (form (cdr forms))
+      (when (symbolp form)
+        (setq form (list form)))
+      (setq body (append form (list body))))
+    body))
+
+;;;; Defined in macroexp.el
+
+(compat-defun macroexpand-1 (form &optional environment)
+  "Perform (at most) one step of macro expansion."
+  :feature macroexp
+  (cond
+   ((consp form)
+    (let* ((head (car form))
+           (env-expander (assq head environment)))
+      (if env-expander
+          (if (cdr env-expander)
+              (apply (cdr env-expander) (cdr form))
+            form)
+        (if (not (and (symbolp head) (fboundp head)))
+            form
+          (let ((def (autoload-do-load (symbol-function head) head 'macro)))
+            (cond
+             ;; Follow alias, but only for macros, otherwise we may end up
+             ;; skipping an important compiler-macro (e.g. cl--block-wrapper).
+             ((and (symbolp def) (macrop def)) (cons def (cdr form)))
+             ((not (consp def)) form)
+             (t
+              (if (eq 'macro (car def))
+                  (apply (cdr def) (cdr form))
+                form))))))))
+   (t form)))
+
+(provide 'compat-25.1)
+;;; compat-25.1.el ends here
diff --git a/compat-26.1.el b/compat-26.1.el
new file mode 100644
index 0000000..1808b14
--- /dev/null
+++ b/compat-26.1.el
@@ -0,0 +1,286 @@
+;;; compat-26.1.el --- Compatibility Layer for Emacs 26.1  -*- 
lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Find here the functionality added in Emacs 26.1, needed by older
+;; versions.
+;;
+;; Do NOT load this library manually.  Instead require `compat'.
+
+;;; Code:
+
+(eval-and-compile (require 'compat-macs))
+
+;;;; Defined in fns.c
+
+(compat-advise assoc (key alist &optional testfn)
+  "Handle TESTFN manually."
+  (if testfn
+      (catch 'found
+        (dolist (ent alist)
+          (when (funcall testfn (car ent) key)
+            (throw 'found ent))))
+    (funcall oldfun key alist)))
+
+(compat-defun mapcan (func sequence)
+  "Apply FUNC to each element of SEQUENCE.
+Concatenate the results by altering them (using `nconc').
+SEQUENCE may be a list, a vector, a boolean vector, or a string."
+  (let (result)
+    (mapc
+     (lambda (elem)
+       (setq result (nconc result (funcall func elem))))
+     sequence)
+    result))
+
+(compat-advise line-number-at-pos (&optional position absolute)
+  "Handle optional argument ABSOLUTE:
+
+If the buffer is narrowed, the return value by default counts the lines
+from the beginning of the accessible portion of the buffer.  But if the
+second optional argument ABSOLUTE is non-nil, the value counts the lines
+from the absolute start of the buffer, disregarding the narrowing."
+  (if absolute
+      (save-restriction
+        (widen)
+        (funcall oldfun position))
+    (funcall oldfun position)))
+
+;;;; Defined in subr.el
+
+(compat-advise alist-get (key alist &optional default remove testfn)
+  "Handle TESTFN manually."
+  :min-version "25.1"                  ;first defined in 25.1
+  :max-version "25.3"                  ;last version without testfn
+  :realname compat--alist-get-handle-testfn
+  (if testfn
+      (catch 'found
+        (dolist (ent alist)
+          (when (and (consp ent) (funcall testfn (car ent) key))
+            (throw 'found (cdr ent))))
+        default)
+    (funcall oldfun key alist default remove testfn)))
+
+(compat-defun alist-get (key alist &optional default remove testfn)
+  "Find the first element of ALIST whose `car' equals KEY and return its `cdr'.
+If KEY is not found in ALIST, return DEFAULT.
+Equality with KEY is tested by TESTFN, defaulting to `eq'."
+  :max-version "24.5"
+  :realname compat--alist-get-full-elisp
+  (ignore remove)
+  (unless testfn (setq testfn #'eq))
+  (catch 'found
+    (dolist (ent alist)
+      (when (and (consp ent) (funcall testfn (car ent) key))
+        (throw 'found (cdr ent))))
+    default))
+
+(compat-defun string-trim-left (string &optional regexp)
+  "Trim STRING of leading string matching REGEXP.
+
+REGEXP defaults to \"[ \\t\\n\\r]+\"."
+  (if (string-match (concat "\\`\\(?:" (or regexp "[ \t\n\r]+") "\\)") string)
+      (substring string (match-end 0))
+    string))
+
+(compat-defun string-trim-right (string &optional regexp)
+  "Trim STRING of trailing string matching REGEXP.
+
+REGEXP defaults to  \"[ \\t\\n\\r]+\"."
+  (let ((i (string-match-p
+            (concat "\\(?:" (or regexp "[ \t\n\r]+") "\\)\\'")
+            string)))
+    (if i (substring string 0 i) string)))
+
+(compat-defun string-trim (string &optional trim-left trim-right)
+  "Trim STRING of leading with and trailing matching TRIM-LEFT and TRIM-RIGHT.
+
+TRIM-LEFT and TRIM-RIGHT default to \"[ \\t\\n\\r]+\"."
+  (compat--string-trim-left
+   (compat--string-trim-right
+    string
+    trim-right)
+   trim-left))
+
+(compat-defun caaar (x)
+  "Return the `car' of the `car' of the `car' of X."
+  (declare (pure t))
+  (car (car (car x))))
+
+(compat-defun caadr (x)
+  "Return the `car' of the `car' of the `cdr' of X."
+  (declare (pure t))
+  (car (car (cdr x))))
+
+(compat-defun cadar (x)
+  "Return the `car' of the `cdr' of the `car' of X."
+  (declare (pure t))
+  (car (cdr (car x))))
+
+(compat-defun caddr (x)
+  "Return the `car' of the `cdr' of the `cdr' of X."
+  (declare (pure t))
+  (car (cdr (cdr x))))
+
+(compat-defun cdaar (x)
+  "Return the `cdr' of the `car' of the `car' of X."
+  (declare (pure t))
+  (cdr (car (car x))))
+
+(compat-defun cdadr (x)
+  "Return the `cdr' of the `car' of the `cdr' of X."
+  (declare (pure t))
+  (cdr (car (cdr x))))
+
+(compat-defun cddar (x)
+  "Return the `cdr' of the `cdr' of the `car' of X."
+  (declare (pure t))
+  (cdr (cdr (car x))))
+
+(compat-defun cdddr (x)
+  "Return the `cdr' of the `cdr' of the `cdr' of X."
+  (declare (pure t))
+  (cdr (cdr (cdr x))))
+
+(compat-defun caaaar (x)
+  "Return the `car' of the `car' of the `car' of the `car' of X."
+  (declare (pure t))
+  (car (car (car (car x)))))
+
+(compat-defun caaadr (x)
+  "Return the `car' of the `car' of the `car' of the `cdr' of X."
+  (declare (pure t))
+  (car (car (car (cdr x)))))
+
+(compat-defun caadar (x)
+  "Return the `car' of the `car' of the `cdr' of the `car' of X."
+  (declare (pure t))
+  (car (car (cdr (car x)))))
+
+(compat-defun caaddr (x)
+  "Return the `car' of the `car' of the `cdr' of the `cdr' of X."
+  (declare (pure t))
+  (car (car (cdr (cdr x)))))
+
+(compat-defun cadaar (x)
+  "Return the `car' of the `cdr' of the `car' of the `car' of X."
+  (declare (pure t))
+  (car (cdr (car (car x)))))
+
+(compat-defun cadadr (x)
+  "Return the `car' of the `cdr' of the `car' of the `cdr' of X."
+  (declare (pure t))
+  (car (cdr (car (cdr x)))))
+
+(compat-defun caddar (x)
+  "Return the `car' of the `cdr' of the `cdr' of the `car' of X."
+  (declare (pure t))
+  (car (cdr (cdr (car x)))))
+
+(compat-defun cadddr (x)
+  "Return the `car' of the `cdr' of the `cdr' of the `cdr' of X."
+  (declare (pure t))
+  (car (cdr (cdr (cdr x)))))
+
+(compat-defun cdaaar (x)
+  "Return the `cdr' of the `car' of the `car' of the `car' of X."
+  (declare (pure t))
+  (cdr (car (car (car x)))))
+
+(compat-defun cdaadr (x)
+  "Return the `cdr' of the `car' of the `car' of the `cdr' of X."
+  (declare (pure t))
+  (cdr (car (car (cdr x)))))
+
+(compat-defun cdadar (x)
+  "Return the `cdr' of the `car' of the `cdr' of the `car' of X."
+  (declare (pure t))
+  (cdr (car (cdr (car x)))))
+
+(compat-defun cdaddr (x)
+  "Return the `cdr' of the `car' of the `cdr' of the `cdr' of X."
+  (declare (pure t))
+  (cdr (car (cdr (cdr x)))))
+
+(compat-defun cddaar (x)
+  "Return the `cdr' of the `cdr' of the `car' of the `car' of X."
+  (declare (pure t))
+  (cdr (cdr (car (car x)))))
+
+(compat-defun cddadr (x)
+  "Return the `cdr' of the `cdr' of the `car' of the `cdr' of X."
+  (declare (pure t))
+  (cdr (cdr (car (cdr x)))))
+
+(compat-defun cdddar (x)
+  "Return the `cdr' of the `cdr' of the `cdr' of the `car' of X."
+  (declare (pure t))
+  (cdr (cdr (cdr (car x)))))
+
+(compat-defun cddddr (x)
+  "Return the `cdr' of the `cdr' of the `cdr' of the `cdr' of X."
+  (declare (pure t))
+  (cdr (cdr (cdr (cdr x)))))
+
+(let ((counter 0))
+  (compat-defun gensym (&optional prefix)
+    "Return a new uninterned symbol.
+The name is made by appending `gensym-counter' to PREFIX.
+PREFIX is a string, and defaults to \"g\"."
+    (let ((num (prog1 counter (setq counter (1+ counter)))))
+      (make-symbol (format "%s%d" (or prefix "g") num)))))
+
+;;;; Defined in files.el
+
+(compat-defun make-nearby-temp-file (prefix &optional dir-flag suffix)
+  "Create a temporary file as close as possible to `default-directory'.
+If PREFIX is a relative file name, and `default-directory' is a
+remote file name or located on a mounted file systems, the
+temporary file is created in the directory returned by the
+function `temporary-file-directory'.  Otherwise, the function
+`make-temp-file' is used.  PREFIX, DIR-FLAG and SUFFIX have the
+same meaning as in `make-temp-file'."
+  (let ((handler (find-file-name-handler
+                  default-directory 'make-nearby-temp-file)))
+    (if (and handler (not (file-name-absolute-p default-directory)))
+        (funcall handler 'make-nearby-temp-file prefix dir-flag suffix)
+      (let ((temporary-file-directory (temporary-file-directory)))
+        (make-temp-file prefix dir-flag suffix)))))
+
+(compat-defun temporary-file-directory ()
+  "The directory for writing temporary files.
+In case of a remote `default-directory', this is a directory for
+temporary files on that remote host.  If such a directory does
+not exist, or `default-directory' ought to be located on a
+mounted file system (see `mounted-file-systems'), the function
+returns `default-directory'.
+For a non-remote and non-mounted `default-directory', the value of
+the variable `temporary-file-directory' is returned."
+  (let ((handler (find-file-name-handler
+                  default-directory 'temporary-file-directory)))
+    (if handler
+        (funcall handler 'temporary-file-directory)
+      (if (string-match mounted-file-systems default-directory)
+          default-directory
+        temporary-file-directory))))
+
+(provide 'compat-26.1)
+;;; compat-26.1.el ends here
diff --git a/compat-27.1.el b/compat-27.1.el
new file mode 100644
index 0000000..dfd7802
--- /dev/null
+++ b/compat-27.1.el
@@ -0,0 +1,271 @@
+;;; compat-27.1.el --- Compatibility Layer for Emacs 27.1  -*- 
lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Find here the functionality added in Emacs 27.1, needed by older
+;; versions.
+;;
+;; Do NOT load this library manually.  Instead require `compat'.
+
+;;; Code:
+
+(eval-and-compile (require 'compat-macs))
+
+;;;; Defined in fns.c
+
+(compat-defun proper-list-p (object)
+  "Return OBJECT's length if it is a proper list, nil otherwise.
+A proper list is neither circular nor dotted (i.e., its last cdr
+is nil)."
+  (catch 'improper
+    (let ((length 0) prev)
+      (while object
+        (unless (consp object)
+          ;; dotted
+          (throw 'improper nil))
+        (when (memq object prev)
+          ;; circular
+          (throw 'improper nil))
+        (setq prev (cons object prev)
+              object (cdr object)
+              length (1+ length)))
+      length)))
+
+(compat-defun string-distance (string1 string2 &optional bytecompare)
+  "Return Levenshtein distance between STRING1 and STRING2.
+The distance is the number of deletions, insertions, and substitutions
+required to transform STRING1 into STRING2.
+If BYTECOMPARE is nil or omitted, compute distance in terms of characters.
+If BYTECOMPARE is non-nil, compute distance in terms of bytes.
+Letter-case is significant, but text properties are ignored."
+  ;; https://en.wikipedia.org/wiki/Levenshtein_distance
+  (let ((s1 (if bytecompare
+                (encode-coding-string string1 'raw-text)
+              (concat string1 "")))
+        (s2 (if bytecompare
+                (encode-coding-string string2 'raw-text)
+              string2)))
+    (let* ((len1 (length s1))
+           (len2 (length s2))
+           (column (make-vector (1+ len1) 0)))
+      (dotimes (y len1)
+        (setf (aref column (1+ y)) y))
+      (dotimes (x len2)
+        (setf (aref column 0) (1+ x))
+        (let ((lastdiag x) olddiag)
+          (dotimes (y len1)
+            (setf olddiag (aref column (1+ y))
+                  (aref column (1+ y))
+                  (min (+ (if (= (aref s1 y) (aref s2 x)) 0 1)
+                          lastdiag)
+                       (1+ (aref column (1+ y)))
+                       (1+ (aref column y)))
+                  lastdiag olddiag))))
+      (aref column len1))))
+
+;;;; Defined in window.c
+
+(compat-advise recenter (&optional arg redisplay)
+  "Handle optional argument REDISPLAY."
+  (funcall oldfun arg)
+  (when (and redisplay recenter-redisplay)
+    (redisplay)))
+
+;;;; Defined in subr.el
+
+(compat-advise setq-local (&rest pairs)
+  "Handle multiple assignments."
+  (unless (zerop (mod (length pairs) 2))
+    (error "PAIRS must have an even number of variable/value members"))
+  (let (body)
+    (while pairs
+      (let* ((sym (pop pairs))
+             (val (pop pairs)))
+        (unless (symbolp sym)
+          (error "Attempting to set a non-symbol: %s" (car pairs)))
+        (push `(set (make-local-variable ,sym) ,val)
+              body)))
+    (cons 'progn (nreverse body))))
+
+(compat-defmacro ignore-error (condition &rest body)
+  "Execute BODY; if the error CONDITION occurs, return nil.
+Otherwise, return result of last form in BODY.
+
+CONDITION can also be a list of error conditions."
+  (declare (debug t) (indent 1))
+  `(condition-case nil (progn ,@body) (,condition nil)))
+
+(compat-defmacro dolist-with-progress-reporter (spec reporter-or-message &rest 
body)
+  "Loop over a list and report progress in the echo area.
+Evaluate BODY with VAR bound to each car from LIST, in turn.
+Then evaluate RESULT to get return value, default nil.
+
+REPORTER-OR-MESSAGE is a progress reporter object or a string.  In the latter
+case, use this string to create a progress reporter.
+
+At each iteration, print the reporter message followed by progress
+percentage in the echo area.  After the loop is finished,
+print the reporter message followed by the word \"done\".
+
+\(fn (VAR LIST [RESULT]) REPORTER-OR-MESSAGE BODY...)"
+  (declare (indent 2) (debug ((symbolp form &optional form) form body)))
+  (let ((prep (make-symbol "--dolist-progress-reporter--"))
+        (count (make-symbol "--dolist-count--"))
+        (list (make-symbol "--dolist-list--")))
+    `(let ((,prep ,reporter-or-message)
+           (,count 0)
+           (,list ,(cadr spec)))
+       (when (stringp ,prep)
+         (setq ,prep (make-progress-reporter ,prep 0 (1- (length ,list)))))
+       (dolist (,(car spec) ,list)
+         ,@body
+         (progress-reporter-update ,prep (setq ,count (1+ ,count))))
+       (progress-reporter-done ,prep)
+       (or ,@(cdr (cdr spec)) nil))))
+
+(compat-defun flatten-tree (tree)
+  "Return a \"flattened\" copy of TREE.
+In other words, return a list of the non-nil terminal nodes, or
+leaves, of the tree of cons cells rooted at TREE.  Leaves in the
+returned list are in the same order as in TREE.
+
+\(flatten-tree \\='(1 (2 . 3) nil (4 5 (6)) 7))
+=> (1 2 3 4 5 6 7)"
+  (let (elems)
+    (while (consp tree)
+      (let ((elem (pop tree)))
+        (while (consp elem)
+          (push (cdr elem) tree)
+          (setq elem (car elem)))
+        (if elem (push elem elems))))
+    (if tree (push tree elems))
+    (nreverse elems)))
+
+(compat-defun xor (cond1 cond2)
+  "Return the boolean exclusive-or of COND1 and COND2.
+If only one of the arguments is non-nil, return it; otherwise
+return nil."
+  (declare (pure t) (side-effect-free error-free))
+  (cond ((not cond1) cond2)
+        ((not cond2) cond1)))
+
+(compat-defvar regexp-unmatchable "\\`a\\`"
+  "Standard regexp guaranteed not to match any string at all."
+  :constant t)
+
+;;;; Defined in files.el
+
+(compat-advise file-size-human-readable (file-size &optional flavor space unit)
+  "Handle the optional third and forth argument:
+
+Optional third argument SPACE is a string put between the number and unit.
+It defaults to the empty string.  We recommend a single space or
+non-breaking space, unless other constraints prohibit a space in that
+position.
+
+Optional fourth argument UNIT is the unit to use.  It defaults to \"B\"
+when FLAVOR is `iec' and the empty string otherwise.  We recommend \"B\"
+in all cases, since that is the standard symbol for byte."
+  (let ((power (if (or (null flavor) (eq flavor 'iec))
+                   1024.0
+                 1000.0))
+        (prefixes '("" "k" "M" "G" "T" "P" "E" "Z" "Y")))
+    (while (and (>= file-size power) (cdr prefixes))
+      (setq file-size (/ file-size power)
+            prefixes (cdr prefixes)))
+    (let* ((prefix (car prefixes))
+           (prefixed-unit (if (eq flavor 'iec)
+                              (concat
+                               (if (string= prefix "k") "K" prefix)
+                               (if (string= prefix "") "" "i")
+                               (or unit "B"))
+                            (concat prefix unit))))
+      (format (if (and (>= (mod file-size 1.0) 0.05)
+                       (< (mod file-size 1.0) 0.95))
+                  "%.1f%s%s"
+                "%.0f%s%s")
+              file-size
+              (if (string= prefixed-unit "") "" (or space ""))
+              prefixed-unit))))
+
+;; TODO provide advice for directory-files-recursively
+
+;;;; Defined in format-spec.el
+
+;; TODO provide advice for format-spec
+
+;;;; Defined in regexp-opt.el
+
+(compat-advise regexp-opt (strings &optional paren)
+  "Handle an empty list of strings."
+  (if (null strings)
+      (let ((re "\\`a\\`"))
+        (cond ((null paren)
+               (concat "\\(?:" re "\\)"))
+              ((stringp paren)
+               (concat paren re "\\)"))
+              ((eq paren 'words)
+               (concat "\\<\\(" re "\\)\\>"))
+              ((eq paren 'symbols)
+               (concat "\\_\\(<" re "\\)\\_>"))
+              ((concat "\\(" re "\\)"))))
+    (funcall oldfun strings paren)))
+
+;;;; Defined in package.el
+
+(declare-function lm-header "lisp-mnt")
+
+(compat-defun package-get-version ()
+  "Return the version number of the package in which this is used.
+Assumes it is used from an Elisp file placed inside the top-level directory
+of an installed ELPA package.
+The return value is a string (or nil in case we can’t find it)."
+  ;; In a sense, this is a lie, but it does just what we want: precompute
+  ;; the version at compile time and hardcodes it into the .elc file!
+  (declare (pure t))
+  ;; Hack alert!
+  (let ((file
+         (or (and (boundp 'byte-compile-current-file) 
byte-compile-current-file)
+             load-file-name
+             buffer-file-name)))
+    (cond
+     ((null file) nil)
+     ;; Packages are normally installed into directories named "<pkg>-<vers>",
+     ;; so get the version number from there.
+     ((string-match
+       
"/[^/]+-\\([0-9]\\(?:[0-9.]\\|pre\\|beta\\|alpha\\|snapshot\\)+\\)/[^/]+\\'"
+       file)
+      (match-string 1 file))
+     ;; For packages run straight from the an elpa.git clone, there's no
+     ;; "-<vers>" in the directory name, so we have to fetch the version
+     ;; the hard way.
+     ((let* ((pkgdir (file-name-directory file))
+             (pkgname (file-name-nondirectory (directory-file-name pkgdir)))
+             (mainfile (expand-file-name (concat pkgname ".el") pkgdir)))
+        (when (file-readable-p mainfile)
+          (require 'lisp-mnt)
+          (with-temp-buffer
+            (insert-file-contents mainfile)
+            (or (lm-header "package-version")
+                (lm-header "version")))))))))
+
+(provide 'compat-27.1)
+;;; compat-27.1.el ends here
diff --git a/compat-28.1.el b/compat-28.1.el
new file mode 100644
index 0000000..d2ff192
--- /dev/null
+++ b/compat-28.1.el
@@ -0,0 +1,456 @@
+;;; compat-28.1.el --- Compatibility Layer for Emacs 28.1  -*- 
lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Find here the functionality added in Emacs 28.1, needed by older
+;; versions.
+;;
+;; Do NOT load this library manually.  Instead require `compat'.
+
+;;; Code:
+
+(eval-and-compile (require 'compat-macs))
+
+;;;; Defined in fns.c
+
+(compat-defun string-search (needle haystack &optional start-pos)
+  "Search for the string NEEDLE in the strign HAYSTACK.
+
+The return value is the position of the first occurrence of
+NEEDLE in HAYSTACK, or nil if no match was found.
+
+The optional START-POS argument says where to start searching in
+HAYSTACK and defaults to zero (start at the beginning).
+It must be between zero and the length of HAYSTACK, inclusive.
+
+Case is always significant and text properties are ignored."
+  (when (and start-pos (< start-pos 0))
+    (signal 'args-out-of-range (list start-pos)))
+  (let ((inhibit-changing-match-data t)
+        (case-fold-search nil))
+    (string-match (regexp-quote needle) haystack start-pos)))
+
+(compat-defun length= (sequence length)
+  "Returns non-nil if SEQUENCE has a length equal to LENGTH."
+  (cond
+   ((null sequence) (zerop length))
+   ((consp sequence)
+    (and (null (nthcdr length sequence))
+         (nthcdr (1- length) sequence)
+         t))
+   ((arrayp sequence)
+    (= (length sequence) length))
+   ((signal 'wrong-type-argument sequence))))
+
+(compat-defun length< (sequence length)
+  "Returns non-nil if SEQUENCE is shorter than LENGTH."
+  (cond
+   ((null sequence) (not (zerop length)))
+   ((listp sequence)
+    (null (nthcdr (1- length) sequence)))
+   ((arrayp sequence)
+    (< (length sequence) length))
+   ((signal 'wrong-type-argument sequence))))
+
+(compat-defun length> (sequence length)
+  "Returns non-nil if SEQUENCE is longer than LENGTH."
+  (cond
+   ((listp sequence)
+    (and (nthcdr length sequence) t))
+   ((arrayp sequence)
+    (> (length sequence) length))
+   ((signal 'wrong-type-argument sequence))))
+
+;;;; Defined in fileio.c
+
+(compat-defun file-name-concat (directory &rest components)
+  "Append COMPONENTS to DIRECTORY and return the resulting string.
+Elements in COMPONENTS must be a string or nil.
+DIRECTORY or the non-final elements in COMPONENTS may or may not end
+with a slash -- if they don’t end with a slash, a slash will be
+inserted before contatenating."
+  (let ((seperator (eval-when-compile
+                     (if (memq system-type '(ms-dos windows-nt cygwin))
+                         "\\" "/")))
+        (last (if components (car (last components)) directory)))
+    (mapconcat (lambda (part)
+                 (if (eq part last)    ;the last component is not modified
+                     last
+                   (replace-regexp-in-string
+                    (concat seperator "+\\'") "" part)))
+               (cons directory components)
+               seperator)))
+
+;;;; Defined in alloc.c
+
+(compat-defun garbage-collect-maybe (_factor)
+  "Call ‘garbage-collect’ if enough allocation happened.
+FACTOR determines what \"enough\" means here: If FACTOR is a
+positive number N, it means to run GC if more than 1/Nth of the
+allocations needed to trigger automatic allocation took place.
+Therefore, as N gets higher, this is more likely to perform a GC.
+Returns non-nil if GC happened, and nil otherwise."
+  ;; Do nothing
+  nil)
+
+;;;; Defined in filelock.c
+
+(compat-advise unlock-buffer ()
+  "Handle `file-error' conditions:
+
+Handles file system errors by calling ‘display-warning’ and
+continuing as if the error did not occur."
+  (condition-case error
+      (funcall oldfun)
+    (file-error
+     (display-warning
+      '(unlock-file)
+      (message "%s, ignored" (error-message-string error))
+      :warning))))
+
+;;;; Defined in characters.c
+
+(compat-advise string-width (string &optional from to)
+  "Handle optional arguments FROM and TO:
+
+Optional arguments FROM and TO specify the substring of STRING to
+consider, and are interpreted as in `substring'."
+  (funcall oldfun (substring string from to)))
+
+;;;; Defined in dired.c
+
+(compat-advise directory-files (directory &optional full match nosort count)
+  "Handle additional optional argument COUNT:
+
+If COUNT is non-nil and a natural number, the function will
+ return COUNT number of file names (if so many are present)."
+  (let ((files (funcall oldfun directory full match nosort)))
+    (when (natnump count)
+      (setf (nthcdr count files) nil))
+    files))
+
+;;;; Defined in subr.el
+
+(compat-defun string-replace (fromstring tostring instring)
+  "Replace FROMSTRING with TOSTRING in INSTRING each time it occurs."
+  (when (equal fromstring "")
+    (signal 'wrong-length-argument '(0)))
+  (let ((case-fold-search nil))
+    (replace-regexp-in-string
+     (regexp-quote fromstring)
+     tostring instring
+     t t)))
+
+(compat-defun always (&rest _arguments)
+  "Do nothing and return t.
+This function accepts any number of ARGUMENTS, but ignores them.
+Also see `ignore'."
+  t)
+
+(compat-defun insert-into-buffer (buffer &optional start end)
+  "Insert the contents of the current buffer into BUFFER.
+If START/END, only insert that region from the current buffer.
+Point in BUFFER will be placed after the inserted text."
+  (let ((current (current-buffer)))
+    (with-current-buffer buffer
+      (insert-buffer-substring current start end))))
+
+(compat-defun replace-regexp-in-region (regexp replacement &optional start end)
+  "Replace REGEXP with REPLACEMENT in the region from START to END.
+The number of replaced occurrences are returned, or nil if REGEXP
+doesn't exist in the region.
+
+If START is nil, use the current point.  If END is nil, use `point-max'.
+
+Comparisons and replacements are done with fixed case.
+
+REPLACEMENT can use the following special elements:
+
+  `\\&' in NEWTEXT means substitute original matched text.
+  `\\N' means substitute what matched the Nth `\\(...\\)'.
+       If Nth parens didn't match, substitute nothing.
+  `\\\\' means insert one `\\'.
+  `\\?' is treated literally."
+  (if start
+      (when (< start (point-min))
+        (error "Start before start of buffer"))
+    (setq start (point)))
+  (if end
+      (when (> end (point-max))
+        (error "End after end of buffer"))
+    (setq end (point-max)))
+  (save-excursion
+    (let ((matches 0)
+          (case-fold-search nil))
+      (goto-char start)
+      (while (re-search-forward regexp end t)
+        (replace-match replacement t)
+        (setq matches (1+ matches)))
+      (and (not (zerop matches))
+           matches))))
+
+(compat-defun buffer-local-boundp (symbol buffer)
+  "Return non-nil if SYMBOL is bound in BUFFER.
+Also see `local-variable-p'."
+  (catch 'fail
+    (condition-case nil
+        (buffer-local-value symbol buffer)
+      (void-variable nil (throw 'fail nil)))
+    t))
+
+(compat-defmacro with-existing-directory (&rest body)
+  "Execute BODY with `default-directory' bound to an existing directory.
+If `default-directory' is already an existing directory, it's not changed."
+  (declare (indent 0) (debug t))
+  (let ((quit (gensym)))
+    `(catch ',quit
+       (dolist (dir (list default-directory
+                          (expand-file-name "~/")
+                          (getenv "TMPDIR")
+                          "/tmp/"
+                          ;; XXX: check if "/" works on non-POSIX
+                          ;; system.
+                          "/"))
+         (when (and dir (file-exists-p dir))
+           (throw ',quit (let ((default-directory dir))
+                           ,@body)))))))
+
+(compat-defmacro dlet (binders &rest body)
+  "Like `let' but using dynamic scoping."
+  (declare (indent 1) (debug let))
+  `(let (_)
+     ,@(mapcar (lambda (binder)
+                 `(defvar ,(if (consp binder) (car binder) binder)))
+               binders)
+     (let ,binders ,@body)))
+
+(compat-defun ensure-list (object)
+  "Return OBJECT as a list.
+If OBJECT is already a list, return OBJECT itself.  If it's
+not a list, return a one-element list containing OBJECT."
+  (if (listp object)
+      object
+    (list object)))
+
+;;;; Defined in subr-x.el
+
+(compat-defun string-clean-whitespace (string)
+  "Clean up whitespace in STRING.
+All sequences of whitespaces in STRING are collapsed into a
+single space character, and leading/trailing whitespace is
+removed."
+  :feature subr-x
+  (let ((blank "[[:blank:]\r\n]+"))
+    (replace-regexp-in-string
+     "^[[:blank:]\r\n]+\\|[[:blank:]\r\n]+$"
+     ""
+     (replace-regexp-in-string
+      blank " " string))))
+
+(compat-defun string-fill (string length)
+  "Clean up whitespace in STRING.
+All sequences of whitespaces in STRING are collapsed into a
+single space character, and leading/trailing whitespace is
+removed."
+  :feature subr-x
+  (with-temp-buffer
+    (insert string)
+    (goto-char (point-min))
+    (let ((fill-column length)
+          (adaptive-fill-mode nil))
+      (fill-region (point-min) (point-max)))
+    (buffer-string)))
+
+(compat-defun string-lines (string &optional omit-nulls)
+  "Split STRING into a list of lines.
+If OMIT-NULLS, empty lines will be removed from the results."
+  :feature subr-x
+  (split-string string "\n" omit-nulls))
+
+(compat-defun string-pad (string length &optional padding start)
+  "Pad STRING to LENGTH using PADDING.
+If PADDING is nil, the space character is used.  If not nil, it
+should be a character.
+
+If STRING is longer than the absolute value of LENGTH, no padding
+is done.
+
+If START is nil (or not present), the padding is done to the end
+of the string, and if non-nil, padding is done to the start of
+the string."
+  :feature subr-x
+  (unless (natnump length)
+    (signal 'wrong-type-argument (list 'natnump length)))
+  (let ((pad-length (- length (length string))))
+    (if (< pad-length 0)
+        string
+      (concat (and start
+                   (make-string pad-length (or padding ?\s)))
+              string
+              (and (not start)
+                   (make-string pad-length (or padding ?\s)))))))
+
+(compat-defun string-chop-newline (string)
+  "Remove the final newline (if any) from STRING."
+  :feature subr-x
+  (if (and (>= (length string) 1) (= (aref string (1- (length string))) ?\n))
+      (substring string 0 -1)
+    string))
+
+(compat-defmacro named-let (name bindings &rest body)
+  "Looping construct taken from Scheme.
+Like `let', bind variables in BINDINGS and then evaluate BODY,
+but with the twist that BODY can evaluate itself recursively by
+calling NAME, where the arguments passed to NAME are used
+as the new values of the bound variables in the recursive invocation."
+  ;; :feature subr-x
+  (declare (indent 2) (debug (symbolp (&rest (symbolp form)) body)))
+  (let* ((fargs (mapcar (lambda (b) (if (consp b) (car b) b)) bindings))
+         (aargs (mapcar (lambda (b) (if (consp b) (cadr b))) bindings))
+         (fn (make-symbol "self"))
+         (macro (lambda (&rest args) `(apply ',fn (list ,@args)))))
+    `(letrec ((,fn (lambda ,fargs . ,(macroexpand-all
+                                      (macroexp-progn body)
+                                      (list (cons name macro))))))
+       (apply ,fn (list ,@aargs)))))
+
+;;;; Defined in files.el
+
+(compat-defun file-name-with-extension (filename extension)
+  "Set the EXTENSION of a FILENAME.
+The extension (in a file name) is the part that begins with the last \".\".
+
+Trims a leading dot from the EXTENSION so that either \"foo\" or
+\".foo\" can be given.
+
+Errors if the FILENAME or EXTENSION are empty, or if the given
+FILENAME has the format of a directory.
+
+See also `file-name-sans-extension'."
+  (let ((extn (string-trim-left extension "[.]")))
+    (cond
+     ((string= filename "")
+      (error "Empty filename"))
+     ((string= extn "")
+      (error "Malformed extension: %s" extension))
+     ((directory-name-p filename)
+      (error "Filename is a directory: %s" filename))
+     (t
+      (concat (file-name-sans-extension filename) "." extn)))))
+
+(compat-defun directory-empty-p (dir)
+  "Return t if DIR names an existing directory containing no other files.
+Return nil if DIR does not name a directory, or if there was
+trouble determining whether DIR is a directory or empty.
+
+Symbolic links to directories count as directories.
+See `file-symlink-p' to distinguish symlinks."
+  (and (file-directory-p dir)
+       (null (directory-files dir nil directory-files-no-dot-files-regexp t))))
+
+;;;; Defined in minibuffer.el
+
+(compat-defun format-prompt (prompt default &rest format-args)
+  "Format PROMPT with DEFAULT.
+If FORMAT-ARGS is nil, PROMPT is used as a plain string.  If
+FORMAT-ARGS is non-nil, PROMPT is used as a format control
+string, and FORMAT-ARGS are the arguments to be substituted into
+it.  See `format' for details.
+
+If DEFAULT is a list, the first element is used as the default.
+If not, the element is used as is.
+
+If DEFAULT is nil or an empty string, no \"default value\" string
+is included in the return value."
+  (concat
+   (if (null format-args)
+       prompt
+     (apply #'format prompt format-args))
+   (and default
+        (or (not (stringp default))
+            (not (null default)))
+        (format " (default %s)"
+                (if (consp default)
+                    (car default)
+                  default)))
+   ": "))
+
+;;;; Defined in windows.el
+
+(compat-advise count-windows (&optional minibuf all-frames)
+  "Handle optional argument ALL-FRAMES:
+
+If ALL-FRAMES is non-nil, count the windows in all frames instead
+just the selected frame."
+  (if all-frames
+      (let ((sum 0))
+        (dolist (frame (frame-list))
+          (with-selected-frame frame
+            (setq sum (+ (funcall oldfun minibuf)
+                         sum))))
+        sum)
+    (funcall oldfun minibuf)))
+
+;;;; Defined in thingatpt.el
+
+(compat-defun thing-at-mouse (event thing &optional no-properties)
+  "Return the THING at mouse click.
+Like `thing-at-point', but tries to use the event
+where the mouse button is clicked to find a thing nearby."
+  :feature thingatpt
+  (save-excursion
+    (mouse-set-point event)
+    (thing-at-point thing no-properties)))
+
+;;;; Defined in macroexp.el
+
+(compat-defun macroexp-file-name ()
+  "Return the name of the file from which the code comes.
+Returns nil when we do not know.
+A non-nil result is expected to be reliable when called from a macro in order
+to find the file in which the macro's call was found, and it should be
+reliable as well when used at the top-level of a file.
+Other uses risk returning non-nil value that point to the wrong file."
+  :feature macroexp
+  (let ((file (car (last current-load-list))))
+    (or (if (stringp file) file)
+        (bound-and-true-p byte-compile-current-file))))
+
+;;;; Defined in env.el
+
+(compat-defmacro with-environment-variables (variables &rest body)
+  "Set VARIABLES in the environent and execute BODY.
+VARIABLES is a list of variable settings of the form (VAR VALUE),
+where VAR is the name of the variable (a string) and VALUE
+is its value (also a string).
+
+The previous values will be be restored upon exit."
+  (declare (indent 1) (debug (sexp body)))
+  (unless (consp variables)
+    (error "Invalid VARIABLES: %s" variables))
+  `(let ((process-environment (copy-sequence process-environment)))
+     ,@(mapcar (lambda (elem)
+                 `(setenv ,(car elem) ,(cadr elem)))
+               variables)
+     ,@body))
+
+(provide 'compat-28.1)
+;;; compat-28.1.el ends here
diff --git a/compat-macs.el b/compat-macs.el
new file mode 100644
index 0000000..c6a2aab
--- /dev/null
+++ b/compat-macs.el
@@ -0,0 +1,245 @@
+;;; compat-macs.el --- Compatibility Macros           -*- lexical-binding: t; 
-*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; These macros are used to define compatibility functions, macros and
+;; advice.
+
+;;; Code:
+
+(defvar compat--disable-defer nil
+  "Prevent compatibility code from being deferred.
+This is used for testing, and should not be set to a non-nil
+value otherwise.")
+
+(defmacro compat--ignore (&rest _)
+  "Ignore all arguments."
+  nil)
+
+(defun compat-generate-common (name def-fn install-fn check-fn attr)
+  "Common code for generating compatibility definitions for NAME.
+The resulting body is constructed by invoking the functions
+DEF-FN (passed the \"realname\" and the version number, returning
+the compatibility definition), the INSTALL-FN (passed the
+\"realname\" and returning the installation code),
+CHECK-FN (passed the \"realname\" and returning a check to see if
+the compatibility definition should be installed).  ATTR is a
+plist used to modify the generated code.  The following
+attributes are handled, all others are ignored:
+
+- :min-version :: Prevent the compatibility definition from begin
+  installed in versions older than indicated (string).
+
+- :max-version :: Prevent the compatibility definition from begin
+  installed in versions newer than indicated (string).
+
+- :feature :: The library the code is supposed to be loaded
+  with (via `eval-after-load').
+
+- :force :: Non-nil means the compatibility code is installed no
+  matter what.
+
+- :version :: Manual specification of the version the compatee
+  code was defined in (string).
+
+- :realname :: Manual specification of a \"realname\" to use for
+  the compatibility definition (symbol)."
+  (let* ((min-version (plist-get attr :min-version))
+         (max-version (plist-get attr :max-version))
+         (feature (plist-get attr :feature))
+         (force (plist-get attr :force))
+         (version (or (plist-get attr :version)
+                      (let ((file (or (and (boundp 'byte-compile-current-file)
+                                           byte-compile-current-file)
+                                      load-file-name
+                                      (buffer-file-name))))
+                        ;; Guess the version from the file the macro is
+                        ;; being defined in.
+                        (and (string-match
+                              
"compat-\\([[:digit:]]+\\.[[:digit:]]+\\)\\.\\(?:elc?\\)\\'"
+                              file)
+                             (match-string 1 file)))))
+         (realname (or (plist-get attr :realname)
+                       (intern (format "compat--%S" name))))
+         (body `(progn
+                  ,(funcall def-fn realname version)
+                  (,@(cond
+                      ((or compat--disable-defer force)
+                       '(progn))
+                      ((or (or (not version)
+                               (version< emacs-version version))
+                           (or (not min-version)
+                               (version<= min-version emacs-version))
+                           (or (not max-version)
+                               (version<= emacs-version max-version)))
+                       '(compat--ignore))
+                      (`(when ,(funcall check-fn realname))))
+                   ,(funcall install-fn realname)))))
+    (if (and feature (not compat--disable-defer))
+        `(eval-after-load ',feature (lambda () ,body))
+      body)))
+
+(defun compat-common-fdefine (type name arglist docstring rest)
+  "Generate compatibility code for a function NAME.
+TYPE is one of `func', for functions and `macro' for macros, and
+`advice' ARGLIST is passed on directly to the definition, and
+DOCSTRING is prepended with a compatibility note.  REST contains
+the remaining definition, that may begin with a property list of
+attributes (see `compat-generate-common')."
+  (let ((body rest))
+    (while (keywordp (car body))
+      (setq body (cddr body)))
+    (compat-generate-common
+     name
+     (lambda (realname version)
+       `(,(cond
+           ((memq type '(func advice)) 'defun)
+           ((eq type 'macro) 'defmacro)
+           ((error "Unknown type")))
+         ,realname ,arglist
+         ;; Prepend compatibility notice to the actual
+         ;; documentation string.
+         ,(let ((type (cond
+                       ((eq type 'func) "function")
+                       ((eq type 'macro) "macro")
+                       ((eq type 'advice) "advice")
+                       ((error "Unknown type")))))
+            (if version
+                (format
+                 "[Compatibility %s for `%S', defined in Emacs %s]\n\n%s"
+                 type name version docstring)
+              (format
+               "[Compatibility %s for `%S']\n\n%s"
+               type name docstring)))
+         ;; Advice may use the implicit variable `oldfun', but
+         ;; to avoid triggering the byte compiler, we make
+         ;; sure the argument is used at least once.
+         ,(and (eq type 'advice) '(ignore oldfun))
+         ;; Insert the actual body now.
+         ,@body))
+     (lambda (realname)
+       (cond
+        ((memq type '(func macro))
+         ;; Functions and macros are installed by
+         ;; aliasing the name of the compatible
+         ;; function to the name of the compatibility
+         ;; function.
+         `(defalias ',name #',realname))
+        ((eq type 'advice)
+         ;; Advice is installed the usual way.
+         `(advice-add ',name :around #',realname))))
+     (lambda (realname)
+       (cond
+        ((memq type '(func macro))
+         `(fboundp ',realname))
+        ((eq type 'advice)
+         ;; TODO: Improve the check if the advice already has been
+         ;;       installed or still has to be added.
+         `(advice-member-p ',realname ',name))))
+     rest)))
+
+(defmacro compat-defun (name arglist docstring &rest rest)
+  "Define NAME with arguments ARGLIST as a compatibility function.
+The function must be documented in DOCSTRING.  REST may begin
+with a plist, that is interpreted by the macro but not passed on
+to the actual function.  See `compat-generate-common' for a
+listing of attributes.
+
+The definition will only be installed, if the version this
+function was defined in, as indicated by the `:version'
+attribute, is greater than the current Emacs version."
+  (declare (debug (&define name (&rest symbolp)
+                           stringp
+                           [&rest keywordp sexp]
+                           def-body))
+           (doc-string 3) (indent 2))
+  (compat-common-fdefine 'func name arglist docstring rest))
+
+(defmacro compat-defmacro (name arglist docstring &rest rest)
+  "Define NAME with arguments ARGLIST as a compatibility macro.
+The macro must be documented in DOCSTRING.  REST may begin
+with a plist, that is interpreted by this macro but not passed on
+to the actual macro.  See `compat-generate-common' for a
+listing of attributes.
+
+The definition will only be installed, if the version this
+function was defined in, as indicated by the `:version'
+attribute, is greater than the current Emacs version."
+  (declare (debug compat-defun) (doc-string 3) (indent 2))
+  (compat-common-fdefine 'macro name arglist docstring rest))
+
+(defmacro compat-advise (name arglist docstring &rest rest)
+  "Define NAME with arguments ARGLIST as a compatibility advice.
+The advice function must be documented in DOCSTRING.  REST may
+begin with a plist, that is interpreted by this macro but not
+passed on to the actual advice function.  See
+`compat-generate-common' for a listing of attributes.  The advice
+wraps the old definition, that is accessible via using the symbol
+`oldfun'.
+
+The advice will only be installed, if the version this function
+was defined in, as indicated by the `:version' attribute, is
+greater than the current Emacs version."
+  (declare (debug compat-defun) (doc-string 3) (indent 2))
+  (compat-common-fdefine 'advice name (cons 'oldfun arglist) docstring rest))
+
+(defmacro compat-defvar (name initval docstring &rest attr)
+  "Declare compatibility variable NAME with initial value INITVAL.
+The obligatory documentation string DOCSTRING must be given.
+
+The remaining arguments ATTR form a plist, modifying the
+behaviour of this macro.  See `compat-generate-common' for a
+listing of attributes.  Furthermore, `compat-defvar' also handles
+the attribute `:local' that either makes the variable permanent
+local with a value of `permanent' or just buffer local with any
+non-nil value."
+  (declare (debug (name form stringp [&rest keywordp sexp]))
+           (doc-string 3) (indent 2))
+  (compat-generate-common
+   name
+   (lambda (realname version)
+     (let ((localp (plist-get attr :local)))
+       `(progn
+          (,(if (plist-get attr :constant) 'defconst 'defvar)
+           ,realname ,initval
+           ;; Prepend compatibility notice to the actual
+           ;; documentation string.
+           ,(if version
+                (format
+                 "[Compatibility variable for `%S', defined in Emacs %s]\n\n%s"
+                 name version docstring)
+              (format
+               "[Compatibility variable for `%S']\n\n%s"
+               name docstring)))
+          ;; Make variable as local if necessary
+          ,(cond
+            ((eq localp 'permanent)
+             `(put ',realname 'permanent-local t))
+            (localp
+             `(make-variable-buffer-local ',realname))))))
+   (lambda (realname)
+     `(defvaralias ',name ',realname))
+   (lambda (realname)
+     `(boundp ',realname))
+   attr))
+
+(provide 'compat-macs)
+;;; compat-macs.el ends here
diff --git a/compat-tests.el b/compat-tests.el
new file mode 100644
index 0000000..703ed5b
--- /dev/null
+++ b/compat-tests.el
@@ -0,0 +1,1195 @@
+;;; compat-tests.el --- Tests for compat.el      -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Package-Requires: ((emacs "28.1"))
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Tests for compatibility functions from compat.el.
+;;
+;; Note that not all functions have tests (yet), for the most part
+;; because it is difficult to create a reproducible environment to
+;; test these in or their tests are trivial -- or a combination of
+;; both.  The current list of untested functions, macros, etc. are:
+;;
+;; - and-let* (`compat--and-let*')
+;; - buffer-local-boundp (`buffer-local-boundp')
+;; - count-windows (`compat--count-windows')
+;; - defmacro (`compat--defmacro')
+;; - directory-empty-p (`compat--directory-empty-p')
+;; - directory-files (`compat--directory-files')
+;; - dlet (`compat--dlet')
+;; - garbage-collect-maybe (`compat--garbage-collect-maybe')
+;; - if-let (`compat--if-let')
+;; - if-let* (`compat--if-let*')
+;; - ignore-error (`compat--ignore-error')
+;; - insert-into-buffer (`compat--insert-into-buffer')
+;; - line-number-at-pos (`compat--line-number-at-pos')
+;; - macroexp-file-name (`compat--macroexp-file-name')
+;; - make-nearby-temp-file (`compat--make-nearby-temp-file')
+;; - named-let (`compat--named-let')
+;; - package-get-version (`compat--package-get-version')
+;; - recenter (`compat--recenter')
+;; - replace-regexp-in-region (`compat--replace-regexp-in-region')
+;; - setq-local (`compat--setq-local')
+;; - temporary-file-directory (`compat--temporary-file-directory')
+;; - thing-at-mouse (`compat--thing-at-mouse')
+;; - thread-first (`compat--thread-first')
+;; - thread-last (`compat--thread-last')
+;; - unlock-buffer (`compat--unlock-buffer')
+;; - when-let (`compat--when-let')
+;; - when-let* (`compat--when-let*')
+;; - with-environment-variables (`compat--with-environment-variables')
+;; - with-existing-directory (`compat--with-existing-directory')
+;; - with-file-modes (`compat--with-file-modes')
+;;
+;; Tests are welcome, but until then anyone working on these functions
+;; should take care to test these manually.
+
+;;; Code:
+
+(require 'ert)
+(eval-and-compile
+  (require 'compat-macs)
+
+  (let ((compat--disable-defer t))
+    (load "compat-24.2.el" nil nil nil t)
+    (load "compat-24.3.el" nil nil nil t)
+    (load "compat-24.4.el" nil nil nil t)
+    (load "compat-25.1.el" nil nil nil t)
+    (load "compat-26.1.el" nil nil nil t)
+    (load "compat-27.1.el" nil nil nil t)
+    (load "compat-28.1.el" nil nil nil t))
+
+  (defvar compat--current-fn nil)
+  (defvar compat--compat-fn nil)
+
+  (defmacro compat--should (result &rest input)
+    "Generate code for test with INPUT evaluating to RESULT."
+    (let ((cfn (or compat--compat-fn
+                   (intern (format "compat--%s" compat--current-fn))))
+          (rfn compat--current-fn))
+      `(progn
+         (should (equal (,cfn ,@input) ,result))
+         (should (equal (,rfn ,@input) ,result)))))
+
+  (defmacro compat--should* (result &rest input)
+    "Generate code for test with INPUT evaluating to RESULT."
+    (let ((cfn (or compat--compat-fn
+                   (intern (format "compat--%s" compat--current-fn))))
+          (rfn compat--current-fn))
+      `(progn
+         (should (equal (funcall (apply-partially #',cfn #',rfn) ,@input) 
,result))
+         (should (equal (,rfn ,@input) ,result)))))
+
+  (defmacro compat--mshould (result &rest input)
+    "Generate code for test with INPUT evaluating to RESULT."
+    (let ((cfn (or compat--compat-fn
+                   (intern (format "compat--%s" compat--current-fn))))
+          (rfn compat--current-fn))
+      `(progn
+         (should (equal (macroexpand-all `(,',cfn ,,@input)) ,result))
+         (should (equal (macroexpand-all `(,',rfn ,,@input)) ,result)))))
+
+  (defmacro compat--error (error &rest input)
+    "Generate code for test FN with INPUT to signal ERROR."
+    (let ((cfn (or compat--compat-fn
+                   (intern (format "compat--%s" compat--current-fn))))
+          (rfn compat--current-fn))
+      `(progn
+         (should-error (,cfn ,@input) :type ',error)
+         (should-error (,rfn ,@input) :type ',error))))
+
+  (defmacro compat--error* (error &rest input)
+    "Generate code for test FN with INPUT to signal ERROR."
+    (let ((cfn (or compat--compat-fn
+                   (intern (format "compat--%s" compat--current-fn))))
+          (rfn compat--current-fn))
+      `(progn
+         (should-error (funcall (apply-partially #',cfn #',rfn) ,@input) :type 
',error)
+         (should-error (,rfn ,@input) :type ',error))))
+
+  ;; FIXME: extract the name of the test out of the ERT-test, instead
+  ;;        of having to re-declare the name of the test redundantly.
+  (defmacro compat-test (fn &rest body)
+    "Set `compat--current-fn' to FN in BODY.
+If FN is a list, the car should be the actual function, and cadr
+the compatibility function."
+    (declare (indent 1))
+    (if (consp fn)
+        (setq compat--current-fn (car fn)
+              compat--compat-fn (cadr fn))
+      (setq compat--current-fn fn
+            compat--compat-fn nil))
+    (macroexp-progn body)))
+
+
+
+(ert-deftest compat-string-search ()
+  "Check if `string-search' was implemented correctly."
+  (compat-test string-search
+    ;; Find needle at the beginning of a haystack:
+    (c-should 0 "a" "abb")
+    ;; Find needle at the begining of a haystack, with more potential
+    ;; needles that could be found:
+    (c-should 0 "a" "abba")
+    ;; Find needle with more than one charachter at the beginning of
+    ;; a line:
+    (c-should 0 "aa" "aabbb")
+    ;; Find a needle midstring:
+    (c-should 1 "a" "bab")
+    ;; Find a needle at the end:
+    (c-should 2 "a" "bba")
+    ;; Find a longer needle midstring:
+    (c-should 1 "aa" "baab")
+    ;; Find a longer needle at the end:
+    (c-should 2 "aa" "bbaa")
+    ;; Find a case-sensitive needle:
+    (c-should 2 "a" "AAa")
+    ;; Find another case-sensitive needle:
+    (c-should 2 "aa" "AAaa")
+    ;; Test regular expression quoting (1):
+    (c-should 5 "." "abbbb.b")
+    ;; Test regular expression quoting (2):
+    (c-should 5 ".*" "abbbb.*b")
+    ;; Attempt to find non-existent needle:
+    (c-should nil "a" "bbb")
+    ;; Attempt to find non-existent needle that has the form of a
+    ;; regular expression:
+    (c-should nil "." "bbb")
+    ;; Handle empty string as needle:
+    (c-should 0 "" "abc")
+    ;; Handle empty string as haystack:
+    (c-should nil "a" "")
+    ;; Handle empty string as needle and haystack:
+    (c-should 0 "" "")
+    ;; Handle START argument:
+    (c-should 3 "a" "abba" 1)
+    ;; Additional test copied from:
+    (c-should 6 "zot" "foobarzot")
+    (c-should 0 "foo" "foobarzot")
+    (c-should nil "fooz" "foobarzot")
+    (c-should nil "zot" "foobarzo")
+    (c-should 0 "ab" "ab")
+    (c-should nil "ab\0" "ab")
+    (c-should 4 "ab" "abababab" 3)
+    (c-should nil "ab" "ababac" 3)
+    (c-should nil "aaa" "aa")
+    (c-should 5
+              (make-string 2 130)
+              (concat "helló" (make-string 5 130 t) "bár"))
+    (c-should 5
+              (make-string 2 127)
+              (concat "helló" (make-string 5 127 t) "bár"))
+    (c-should 1 "\377" "a\377ø")
+    (c-should 1 "\377" "a\377a")
+    (c-should nil (make-string 1 255) "a\377ø")
+    (c-should nil (make-string 1 255) "a\377a")
+    (c-should 3 "fóo" "zotfóo")
+    (c-should 2 (string-to-multibyte "\377") "ab\377c")
+    (c-should nil "\303" "aøb")
+    (c-should nil "\270" "aøb")
+    (c-should nil "ø" "\303\270")
+    (c-should nil "ø" (make-string 32 ?a))
+    (c-should nil "ø" (string-to-multibyte (make-string 32 ?a)))
+    (c-should 14 "o" (string-to-multibyte
+                      (apply #'string (number-sequence ?a ?z))))
+    (c-should 2 "a\U00010f98z" "a\U00010f98a\U00010f98z")
+    (c-error (args-out-of-range -1) "a" "abc" -1)
+    (c-error (args-out-of-range 4) "a" "abc" 4)
+    (c-error (args-out-of-range 100000000000)
+             "a" "abc" 100000000000)
+    (c-should nil "a" "aaa" 3)
+    (c-should nil "aa" "aa" 1)
+    (c-should nil "\0" "")
+    (c-should 0 "" "")
+    (c-error (args-out-of-range 1) "" "" 1)
+    (c-should 0 "" "abc")
+    (c-should 2 "" "abc" 2)
+    (c-should 3 "" "abc" 3)
+    (c-error (args-out-of-range 4) "" "abc" 4)
+    (c-error (args-out-of-range -1) "" "abc" -1)
+    (c-should nil "ø" "foo\303\270")
+    (c-should nil "\303\270" "ø")
+    (c-should nil "\370" "ø")
+    (c-should nil (string-to-multibyte "\370") "ø")
+    (c-should nil "ø" "\370")
+    (c-should nil "ø" (string-to-multibyte "\370"))
+    (c-should nil "\303\270" "\370")
+    (c-should nil (string-to-multibyte "\303\270") "\370")
+    (c-should nil "\303\270" (string-to-multibyte "\370"))
+    (c-should nil
+              (string-to-multibyte "\303\270")
+              (string-to-multibyte "\370"))
+    (c-should nil "\370" "\303\270")
+    (c-should nil (string-to-multibyte "\370") "\303\270")
+    (c-should nil "\370" (string-to-multibyte "\303\270"))
+    (c-should nil
+              (string-to-multibyte "\370")
+              (string-to-multibyte "\303\270"))
+    (c-should 2
+              (string-to-multibyte "o\303\270")
+              "foo\303\270")
+    (c-should 3 "\303\270" "foo\303\270")))
+
+(ert-deftest compat-string-replace ()
+  "Check if `string-replace' was implemented correctly."
+  (compat-test string-replace
+    (c-should "bba" "aa" "bb" "aaa")
+    (c-should "AAA" "aa" "bb" "AAA")
+    ;; Additional test copied from subr-tests.el:
+    (c-should "zot" "foo" "bar" "zot")
+    (c-should "barzot" "foo" "bar" "foozot")
+    (c-should "barbarzot" "foo" "bar" "barfoozot")
+    (c-should "barfoobar" "zot" "bar" "barfoozot")
+    (c-should "barfoobarot" "z" "bar" "barfoozot")
+    (c-should "zat" "zot" "bar" "zat")
+    (c-should "zat" "azot" "bar" "zat")
+    (c-should "bar" "azot" "bar" "azot")
+    (c-should "foozotbar" "azot" "bar" "foozotbar")
+    (c-should "labarbarbarzot" "fo" "bar" "lafofofozot")
+    (c-should "axb" "\377" "x" "a\377b")
+    (c-should "axø" "\377" "x" "a\377ø")
+    (c-should "axb" (string-to-multibyte "\377") "x" "a\377b")
+    (c-should "axø" (string-to-multibyte "\377") "x" "a\377ø")
+    (c-should "ANAnas" "ana" "ANA" "ananas")
+    (c-should "" "a" "" "")
+    (c-should "" "a" "" "aaaaa")
+    (c-should "" "ab" "" "ababab")
+    (c-should "ccc" "ab" "" "abcabcabc")
+    (c-should "aaaaaa" "a" "aa" "aaa")
+    (c-should "defg" "abc" "defg" "abc")
+    (c-error wrong-length-argument "" "x" "abc")))
+
+(ert-deftest compat-length= ()
+  "Check if `string-length=' was implemented correctly."
+  (compat-test length=
+    (c-should t '() 0)                  ;empty list
+    (c-should t '(1) 1)                        ;single element
+    (c-should t '(1 2 3) 3)             ;multiple elements
+    (c-should nil '(1 2 3) 2)           ;less than
+    (c-should nil '(1) 0)
+    (c-should nil '(1 2 3) 4)           ;more than
+    (c-should nil '(1) 2)
+    (c-should nil '() 1)
+    (c-should t [] 0)                   ;empty vector
+    (c-should t [1] 1)                 ;single element vector
+    (c-should t [1 2 3] 3)              ;multiple element vector
+    (c-should nil [1 2 3] 2)            ;less than
+    (c-should nil [1 2 3] 4)            ;more than
+    (c-error wrong-type-argument 3 nil)))
+
+(ert-deftest compat-length< ()
+  "Check if `length<' was implemented correctly."
+  (compat-test length<
+    (c-should nil '(1) 0)               ;single element
+    (c-should nil '(1 2 3) 2)           ;multiple elements
+    (c-should nil '(1 2 3) 3)           ;equal length
+    (c-should nil '(1) 1)
+    (c-should t '(1 2 3) 4)             ;more than
+    (c-should t '(1) 2)
+    (c-should t '() 1)
+    (c-should nil [1] 0)                ;single element vector
+    (c-should nil [1 2 3] 2)            ;multiple element vector
+    (c-should nil [1 2 3] 3)            ;equal length
+    (c-should t [1 2 3] 4)              ;more than
+    (c-error wrong-type-argument 3 nil)))
+
+(ert-deftest compat-length> ()
+  "Check if `length>' was implemented correctly."
+  (compat-test length>
+    (c-should t '(1) 0)                        ;single element
+    (c-should t '(1 2 3) 2)             ;multiple elements
+    (c-should nil '(1 2 3) 3)           ;equal length
+    (c-should nil '(1) 1)
+    (c-should nil '(1 2 3) 4)           ;more than
+    (c-should nil '(1) 2)
+    (c-should nil '() 1)
+    (c-should t [1] 0)                 ;single element vector
+    (c-should t [1 2 3] 2)              ;multiple element vector
+    (c-should nil [1 2 3] 3)            ;equal length
+    (c-should nil [1 2 3] 4)            ;more than
+    (c-error wrong-type-argument 3 nil)))
+
+(ert-deftest compat-always ()
+  "Check if `always' was implemented correctly."
+  (compat-test always
+    (c-should t)                        ;no arguments
+    (c-should t 1)                      ;single argument
+    (c-should t 1 2 3 4)))              ;multiple arguments
+
+(ert-deftest compat-insert-into-buffer ()
+  "Check if `insert-into-buffer' was implemented correctly."
+  ;; Without optional arguments
+  (with-temp-buffer
+    (let ((other (current-buffer)))
+      (insert "abc")
+      (with-temp-buffer
+       (insert "def")
+       (c-insert-into-buffer other))
+      (should (string= (buffer-string) "abcdef"))))
+  (when (fboundp 'insert-into-buffer)
+    (with-temp-buffer
+      (let ((other (current-buffer)))
+       (insert "abc")
+       (with-temp-buffer
+         (insert "def")
+         (insert-into-buffer other))
+       (should (string= (buffer-string) "abcdef")))))
+  ;; With one optional argument
+  (with-temp-buffer
+    (let ((other (current-buffer)))
+      (insert "abc")
+      (with-temp-buffer
+       (insert "def")
+       (c-insert-into-buffer other 2))
+      (should (string= (buffer-string) "abcef"))))
+  (when (fboundp 'insert-into-buffer)
+    (with-temp-buffer
+      (let ((other (current-buffer)))
+       (insert "abc")
+       (with-temp-buffer
+         (insert "def")
+         (insert-into-buffer other 2))
+       (should (string= (buffer-string) "abcef")))))
+  ;; With two optional arguments
+  (with-temp-buffer
+    (let ((other (current-buffer)))
+      (insert "abc")
+      (with-temp-buffer
+       (insert "def")
+       (c-insert-into-buffer other 2 3))
+      (should (string= (buffer-string) "abce"))))
+  (when (fboundp 'insert-into-buffer)
+    (with-temp-buffer
+      (let ((other (current-buffer)))
+       (insert "abc")
+       (with-temp-buffer
+         (insert "def")
+         (insert-into-buffer other 2 3))
+       (should (string= (buffer-string) "abce"))))))
+
+(ert-deftest compat-file-name-with-extension ()
+  "Check if `file-name-with-extension' was implemented correctly."
+  (compat-test file-name-with-extension
+    (c-should "file.ext" "file" "ext")
+    (c-should "file.ext" "file" ".ext")
+    (c-should "file.ext" "file." ".ext")
+    (c-should "file..ext" "file.." ".ext")
+    (c-should "file..ext" "file." "..ext")
+    (c-should "file...ext" "file.." "..ext")
+    (c-should "/abs/file.ext" "/abs/file" "ext")
+    (c-should "/abs/file.ext" "/abs/file" ".ext")
+    (c-should "/abs/file.ext" "/abs/file." ".ext")
+    (c-should "/abs/file..ext" "/abs/file.." ".ext")
+    (c-should "/abs/file..ext" "/abs/file." "..ext")
+    (c-should "/abs/file...ext" "/abs/file.." "..ext")
+    (c-error error "file" "")
+    (c-error error '"" "ext")
+    (c-error error "file" "")
+    (c-error error "rel/" "ext")
+    (c-error error "/abs/" "ext")))
+
+(ert-deftest compat-file-name-with-extension ()
+  "Check if `file-name-with-extension' was implemented correctly."
+  (compat-test file-name-with-extension
+    (c-should "file.ext" "file" "ext")
+    (c-should "file.ext" "file" ".ext")
+    (c-should "file.ext" "file." ".ext")
+    (c-should "file..ext" "file.." ".ext")
+    (c-should "file..ext" "file." "..ext")
+    (c-should "file...ext" "file.." "..ext")
+    (c-should "/abs/file.ext" "/abs/file" "ext")
+    (c-should "/abs/file.ext" "/abs/file" ".ext")
+    (c-should "/abs/file.ext" "/abs/file." ".ext")
+    (c-should "/abs/file..ext" "/abs/file.." ".ext")
+    (c-should "/abs/file..ext" "/abs/file." "..ext")
+    (c-should "/abs/file...ext" "/abs/file.." "..ext")
+    (c-error error "file" "")
+    (c-error error "" "ext")
+    (c-error error "file" "")
+    (c-error error "rel/" "ext")
+    (c-error error "/abs/" "ext")))
+
+(ert-deftest compat-string-width ()
+  "Check if `string-width' was implemented correctly."
+  (compat-test string-width
+    (c-should* 0 "")
+    (c-should* 3 "abc")                        ;no argument
+    (c-should* 5 "abcあ")
+    (c-should* (1+ tab-width) "a       ")
+    (c-should* 2 "abc" 1)               ;with from
+    (c-should* 4 "abcあ" 1)
+    (c-should* tab-width "a    " 1)
+    (c-should* 2 "abc" 0 2)             ;with to
+    (c-should* 3 "abcあ" 0 3)
+    (c-should* 1 "a    " 0 1)
+    (c-should* 1 "abc" 1 2)             ;with from and to
+    (c-should* 2 "abcあ" 3 4)
+    (c-should* 0 "a    " 1 1)))
+
+(ert-deftest compat-ensure-list ()
+  "Check if `ensure-list' was implemented correctly."
+  (compat-test ensure-list
+    (c-should nil nil)                        ;empty list
+    (c-should '(1) '(1))                        ;single element list
+    (c-should '(1 2 3) '(1 2 3))                ;multiple element list
+    (c-should '(1) 1)))                          ;atom
+
+(ert-deftest compat-proper-list-p ()
+  "Check if `proper-list-p' was implemented correctly."
+  (compat-test proper-list-p
+    (c-should 0 ())                            ;empty list
+    (c-should 1 '(1))                          ;single element
+    (c-should 3 '(1 2 3))                      ;multiple elements
+    (c-should nil '(1 . 2))                    ;cons
+    (c-should nil '(1 2 . 3))                  ;dotted
+    (c-should nil (let ((l (list 1 2 3)))              ;circular
+                    (setf (nthcdr 3 l) l)
+                    l))))
+
+(ert-deftest compat-flatten-tree ()
+  "Check if `flatten-tree' was implemented correctly."
+  (compat-test flatten-tree
+    ;; Example from docstring:
+    (c-should '(1 2 3 4 5 6 7) '(1 (2 . 3) nil (4 5 (6)) 7))
+    ;; Trivial example
+    (c-should nil ())
+    ;; Simple examples
+    (c-should '(1) '(1))
+    (c-should '(1 2) '(1 2))
+    (c-should '(1 2 3) '(1 2 3))
+    ;; Regular sublists
+    (c-should '(1) '((1)))
+    (c-should '(1 2) '((1) (2)))
+    (c-should '(1 2 3) '((1) (2) (3)))
+    ;; Complex examples
+    (c-should '(1) '(((((1))))))
+    (c-should '(1 2 3 4) '((1) nil 2 ((3 4))))
+    (c-should '(1 2 3 4) '(((1 nil)) 2 (((3 nil nil) 4))))))
+
+(ert-deftest compat-xor ()
+  "Check if `xor' was implemented correctly."
+  (compat-test xor
+    (c-should t t nil)
+    (c-should t nil t)
+    (c-should nil nil nil)
+    (c-should nil t t)))
+
+(ert-deftest compat- ()
+  "Check if `string-distance' was implemented correctly."
+  (compat-test string-distance
+    (c-should 3 "kitten" "sitting")     ;from wikipedia
+    (c-should 0 "" "")                  ;trivial examples
+    (c-should 0 "a" "a")
+    (c-should 1 "" "a")
+    (c-should 1 "b" "a")
+    (c-should 2 "aa" "bb")
+    (c-should 2 "aa" "bba")
+    (c-should 2 "aaa" "bba")
+    (c-should 3 "a" "あ" t)             ;byte example
+    (c-should 1 "a" "あ")))
+
+(ert-deftest compat-regexp-unmatchable ()
+  "Check if `string-distance' was implemented correctly."
+  (dolist (str '(""                     ;empty string
+                 "a"                    ;simple string
+                 "aaa"                  ;longer string
+                 ))
+    (should-not (string-match-p (with-no-warnings c-regexp-unmatchable) str))
+    (should-not (string-match-p regexp-unmatchable str))))
+
+(ert-deftest compat-regexp-opt ()
+  "Check if `regexp-opt' advice was defined correctly."
+  (compat-test regexp-opt
+    ;; Ensure `compat--regexp-opt' doesn't change the existing
+    ;; behaviour:
+    (c-should* (regexp-opt '("a" "b" "c")) '("a" "b" "c"))
+    (c-should* (regexp-opt '("abc" "def" "ghe")) '("abc" "def" "ghe"))
+    (c-should* (regexp-opt '("a" "b" "c") 'words) '("a" "b" "c") 'words)
+    ;; Test empty list:
+    (c-should* "\\(?:\\`a\\`\\)" '())
+    (c-should* "\\<\\(\\`a\\`\\)\\>" '() 'words))
+  (let ((unmatchable (regexp-opt '())))
+    (dolist (str '(""                   ;empty string
+                   "a"                  ;simple string
+                   "aaa"                ;longer string
+                   ))
+      (should-not (string-match-p unmatchable str)))))
+
+(ert-deftest compat-assoc ()
+  "Check if `assoc' advice was advised correctly."
+  (compat-test assoc
+    ;; Fallback behaviour:
+    (c-should* nil 1 nil)               ;empty list
+    (c-should* '(1) 1 '((1)))            ;single element list
+    (c-should* nil 1 '(1))
+    (c-should* '(2) 2 '((1) (2) (3)))    ;multiple element list
+    (c-should* nil 2 '(1 2 3))
+    (c-should* '(2) 2 '(1 (2) 3))
+    (c-should* nil 2 '((1) 2 (3)))
+    (c-should* '(1) 1 '((3) (2) (1)))
+    (c-should* '("a") "a" '(("a") ("b") ("c")))  ;non-primitive elements
+    (c-should* '("a" 0) "a" '(("c" . "a") "b" ("a" 0)))
+    ;; With testfn (advised behaviour):
+    (c-should* '(1) 3 '((10) (4) (1) (9)) #'<)
+    (c-should* '("a") "b" '(("c") ("a") ("b")) #'string-lessp)
+    (c-should* '("b") "a" '(("a") ("a") ("b"))
+               (lambda (s1 s2) (not (string= s1 s2))))
+    (c-should*
+     '("\\.el\\'" . emacs-lisp-mode)
+     "file.el"
+     '(("\\.c\\'" . c-mode)
+       ("\\.p\\'" . pascal-mode)
+       ("\\.el\\'" . emacs-lisp-mode)
+       ("\\.awk\\'" . awk-mode))
+     #'string-match-p)))
+
+(ert-deftest compat-alist-get-1 ()
+  "Check if `alist-get' was advised correctly."
+  (compat-test (alist-get compat--alist-get-handle-testfn)
+    ;; Fallback behaviour:
+    (c-should* nil 1 nil)                      ;empty list
+    (c-should* 'a 1 '((1 . a)))                  ;single element list
+    (c-should* nil 1 '(1))
+    (c-should* 'b 2 '((1 . a) (2 . b) (3 . c)))  ;multiple element list
+    (c-should* nil 2 '(1 2 3))
+    (c-should* 'b 2 '(1 (2 . b) 3))
+    (c-should* nil 2 '((1 . a) 2 (3 . c)))
+    (c-should* 'a 1 '((3 . c) (2 . b) (1 . a)))
+    (c-should* nil "a" '(("a" . 1) ("b" . 2) ("c" . 3)))  ;non-primitive 
elements
+
+    ;; With testfn (advised behaviour):
+    (c-should* 1 "a" '(("a" . 1) ("b" . 2) ("c" . 3)) nil nil #'equal)
+    (c-should* 1 3 '((10 . 10) (4 . 4) (1 . 1) (9 . 9)) nil nil #'<)
+    (c-should* '(a) "b" '(("c" c) ("a" a) ("b" b)) nil nil #'string-lessp)
+    (c-should* 'c "a" '(("a" . a) ("a" . b) ("b" . c)) nil nil
+               (lambda (s1 s2) (not (string= s1 s2))))
+    (c-should* 'emacs-lisp-mode
+               "file.el"
+               '(("\\.c\\'" . c-mode)
+                 ("\\.p\\'" . pascal-mode)
+                 ("\\.el\\'" . emacs-lisp-mode)
+                 ("\\.awk\\'" . awk-mode))
+               nil nil #'string-match-p)
+    (c-should* 'd 0 '((1 . a) (2 . b) (3 . c)) 'd) ;default value
+    (c-should* 'd 2 '((1 . a) (2 . b) (3 . c)) 'd nil #'ignore)))
+
+(ert-deftest compat-alist-get-2 ()
+  "Check if `alist-get' was implemented correctly."
+  (compat-test (alist-get compat--alist-get-full-elisp)
+    ;; Fallback behaviour:
+    (c-should nil 1 nil)                      ;empty list
+    (c-should 'a 1 '((1 . a)))                  ;single element list
+    (c-should nil 1 '(1))
+    (c-should 'b 2 '((1 . a) (2 . b) (3 . c)))  ;multiple element list
+    (c-should nil 2 '(1 2 3))
+    (c-should 'b 2 '(1 (2 . b) 3))
+    (c-should nil 2 '((1 . a) 2 (3 . c)))
+    (c-should 'a 1 '((3 . c) (2 . b) (1 . a)))
+    (c-should nil "a" '(("a" . 1) ("b" . 2) ("c" . 3)))  ;non-primitive 
elements
+
+    ;; With testfn (advised behaviour):
+    (c-should 1 "a" '(("a" . 1) ("b" . 2) ("c" . 3)) nil nil #'equal)
+    (c-should 1 3 '((10 . 10) (4 . 4) (1 . 1) (9 . 9)) nil nil #'<)
+    (c-should '(a) "b" '(("c" c) ("a" a) ("b" b)) nil nil #'string-lessp)
+    (c-should 'c "a" '(("a" . a) ("a" . b) ("b" . c)) nil nil
+              (lambda (s1 s2) (not (string= s1 s2))))
+    (c-should 'emacs-lisp-mode
+              "file.el"
+              '(("\\.c\\'" . c-mode)
+                ("\\.p\\'" . pascal-mode)
+                ("\\.el\\'" . emacs-lisp-mode)
+                ("\\.awk\\'" . awk-mode))
+              nil nil #'string-match-p)
+    (c-should 'd 0 '((1 . a) (2 . b) (3 . c)) 'd) ;default value
+    (c-should 'd 2 '((1 . a) (2 . b) (3 . c)) 'd nil #'ignore)))
+
+(ert-deftest compat-string-trim-left ()
+  "Check if `string-trim-left' was implemented correctly."
+  (compat-test string-trim-left'
+    (c-should "" "")                          ;empty string
+    (c-should "a" "a")                        ;"full" string
+    (c-should "aaa" "aaa")
+    (c-should "へっろ" "へっろ")
+    (c-should "hello world" "hello world")
+    (c-should "a " "a ")                        ;right trailing
+    (c-should "aaa " "aaa ")
+    (c-should "a    " "a    ")
+    (c-should "a\t\t" "a\t\t")
+    (c-should "a\n  \t" "a\n  \t")
+    (c-should "a" " a")                        ;left trailing
+    (c-should "aaa" " aaa")
+    (c-should "a" "a")
+    (c-should "a" "\t\ta")
+    (c-should "a" "\n  \ta")
+    (c-should "a " " a ")                        ;both trailing
+    (c-should "aaa  " " aaa  ")
+    (c-should "a\t\n" "\t\ta\t\n")
+    (c-should "a  \n" "\n  \ta  \n")))
+
+(ert-deftest compat-string-trim-right ()
+  "Check if `string-trim-right' was implemented correctly."
+  (compat-test string-trim-right
+    (c-should "" "")                          ;empty string
+    (c-should "a" "a")                        ;"full" string
+    (c-should "aaa" "aaa")
+    (c-should "へっろ" "へっろ")
+    (c-should "hello world" "hello world")
+    (c-should "a" "a")                      ;right trailing
+    (c-should "aaa" "aaa")
+    (c-should "a" "a    ")
+    (c-should "a" "a\t\t")
+    (c-should "a" "a\n  \t")
+    (c-should " a" " a")                       ;left trailing
+    (c-should " aaa" " aaa")
+    (c-should "a" "a")
+    (c-should "\t\ta" "\t\ta")
+    (c-should "\n  \ta" "\n  \ta")
+    (c-should " a" " a ")                        ;both trailing
+    (c-should " aaa" " aaa")
+    (c-should "\t\ta" "\t\ta\t\n")
+    (c-should "\n  \ta" "\n  \ta  \n")))
+
+(ert-deftest compat-string-trim ()
+  "Check if `string-trim' was implemented correctly."
+  (compat-test string-trim
+    (c-should "" "")                          ;empty string
+    (c-should "a" "a")                        ;"full" string
+    (c-should "aaa" "aaa")
+    (c-should "へっろ" "へっろ")
+    (c-should "hello world" "hello world")
+    (c-should "a" "a ")                       ;right trailing
+    (c-should "aaa" "aaa ")
+    (c-should "a" "a    ")
+    (c-should "a" "a\t\t")
+    (c-should "a" "a\n  \t")
+    (c-should "a" " a")                       ;left trailing
+    (c-should "aaa" " aaa")
+    (c-should "a" "a")
+    (c-should "a" "\t\ta")
+    (c-should "a" "\n  \ta")
+    (c-should "a" " a ")                      ;both trailing
+    (c-should "aaa" " aaa  ")
+    (c-should "t\ta" "t\ta\t\n")
+    (c-should "a" "\n  \ta  \n")))
+
+(ert-deftest compat-mapcan ()
+  "Check if `mapcan' was implemented correctly."
+  (compat-test mapcan
+    (c-should nil #'identity nil)
+    (c-should (list 1)
+              #'identity
+              (list (list 1)))
+    (c-should (list 1 2 3 4)
+              #'identity
+              (list (list 1) (list 2 3) (list 4)))
+    (c-should (list (list 1) (list 2 3) (list 4))
+              #'list
+              (list (list 1) (list 2 3) (list 4)))
+    (c-should (list 1 2 3 4)
+              #'identity
+              (list (list 1) (list) (list 2 3) (list 4)))
+    (c-should (list (list 1) (list) (list 2 3) (list 4))
+              #'list
+              (list (list 1) (list) (list 2 3) (list 4)))
+    (c-should (list)
+              #'identity
+              (list (list) (list) (list) (list)))))
+
+;; Note: as the cXXX+r implementations are relatively trivial, their
+;; tests are not as extensive.
+
+(defvar compat-cXXXr-test
+  '(((a . b) . (c . d)) . ((e . f) . (g . h)))
+  "Testcase for cXXXr functions.")
+
+(defvar compat-cXXXXr-test
+  '((((a . b) . (c . d)) . ((e . f) . (g . h))) .
+    (((i . j) . (k . l)) . ((m . j) . (o . p))))
+  "Testcase for cXXXXr functions.")
+
+(ert-deftest compat-caaar ()
+  "Check if `caaar' was implemented correctly."
+  (compat-test caaar
+    (c-should nil ())
+    (c-should 'a compat-cXXXr-test)))
+
+(ert-deftest compat-caadr ()
+  "Check if `caadr' was implemented correctly."
+  (compat-test caadr
+    (c-should nil ())
+    (c-should 'e compat-cXXXr-test)))
+
+(ert-deftest compat-cadar ()
+  "Check if `cadar' was implemented correctly."
+  (compat-test cadar
+    (c-should nil ())
+    (c-should 'c compat-cXXXr-test)))
+
+(ert-deftest compat-caddr ()
+  "Check if `caddr' was implemented correctly."
+  (compat-test caddr
+    (c-should nil ())
+    (c-should 'g compat-cXXXr-test)))
+
+(ert-deftest compat-cdaar ()
+  "Check if `cdaar' was implemented correctly."
+  (compat-test cdaar
+    (c-should nil ())
+    (c-should 'b compat-cXXXr-test)))
+
+(ert-deftest compat-cdadr ()
+  "Check if `cdadr' was implemented correctly."
+  (compat-test cdadr
+    (c-should nil ())
+    (c-should 'f compat-cXXXr-test)))
+
+(ert-deftest compat-cddar ()
+  "Check if `cddar' was implemented correctly."
+  (compat-test cddar
+    (c-should nil ())
+    (c-should 'd compat-cXXXr-test)))
+
+(ert-deftest compat-cdddr ()
+  "Check if `cdddr' was implemented correctly."
+  (compat-test cdddr
+    (c-should nil ())
+    (c-should 'h compat-cXXXr-test)
+    #'cdddr))
+
+(ert-deftest compat-caaaar ()
+  "Check if `caaaar' was implemented correctly."
+  (compat-test caaaar
+    (c-should nil ())
+    (c-should 'a compat-cXXXXr-test)))
+
+(ert-deftest compat-caaadr ()
+  "Check if `caaadr' was implemented correctly."
+  (compat-test caaadr
+    (c-should nil ())
+    (c-should 'i compat-cXXXXr-test)))
+
+(ert-deftest compat-caadar ()
+  "Check if `caadar' was implemented correctly."
+  (compat-test caadar
+    (c-should nil ())
+    (c-should 'e compat-cXXXXr-test)))
+
+(ert-deftest compat-caaddr ()
+  "Check if `caaddr' was implemented correctly."
+  (compat-test caaddr
+    (c-should nil ())
+    (c-should 'm compat-cXXXXr-test)))
+
+(ert-deftest compat-cadaar ()
+  "Check if `cadaar' was implemented correctly."
+  (compat-test cadaar
+    (c-should nil ())
+    (c-should 'c compat-cXXXXr-test)))
+
+(ert-deftest compat-cadadr ()
+  "Check if `cadadr' was implemented correctly."
+  (compat-test cadadr
+    (c-should nil ())
+    (c-should 'k compat-cXXXXr-test)))
+
+(ert-deftest compat-caddar ()
+  "Check if `caddar' was implemented correctly."
+  (compat-test caddar
+    (c-should nil ())
+    (c-should 'g compat-cXXXXr-test)))
+
+(ert-deftest compat-cadddr ()
+  "Check if `cadddr' was implemented correctly."
+  (compat-test cadddr
+    (c-should nil ())
+    (c-should 'o compat-cXXXXr-test)))
+
+(ert-deftest compat-cdaaar ()
+  "Check if `cdaaar' was implemented correctly."
+  (compat-test cdaaar
+    (c-should nil ())
+    (c-should 'b compat-cXXXXr-test)))
+
+(ert-deftest compat-cdaadr ()
+  "Check if `cdaadr' was implemented correctly."
+  (compat-test cdaadr
+    (c-should nil ())
+    (c-should 'j compat-cXXXXr-test)))
+
+(ert-deftest compat-cdadar ()
+  "Check if `cdadar' was implemented correctly."
+  (compat-test cdadar
+    (c-should nil ())
+    (c-should 'f compat-cXXXXr-test)))
+
+(ert-deftest compat-cdaddr ()
+  "Check if `cdaddr' was implemented correctly."
+  (compat-test cdaddr
+    (c-should nil ())
+    (c-should 'j compat-cXXXXr-test)))
+
+(ert-deftest compat-cddaar ()
+  "Check if `cddaar' was implemented correctly."
+  (compat-test cddaar
+    (c-should nil ())
+    (c-should 'd compat-cXXXXr-test)))
+
+(ert-deftest compat-cddadr ()
+  "Check if `cddadr' was implemented correctly."
+  (compat-test cddadr
+    (c-should nil ())
+    (c-should 'l compat-cXXXXr-test)))
+
+(ert-deftest compat-cdddar ()
+  "Check if `cdddar' was implemented correctly."
+  (compat-test cdddar
+    (c-should nil ())
+    (c-should 'h compat-cXXXXr-test)))
+
+(ert-deftest compat-string-greaterp ()
+  "Check if `string-greaterp' was implemented correctly."
+  (compat-test string-greaterp
+    (c-should t "b" "a")
+    (c-should nil "a" "b")
+    (c-should t "aaab" "aaaa")
+    (c-should nil "aaaa" "aaab")))
+
+(ert-deftest compat-sort ()
+  "Check if `sort' was advised correctly."
+  (compat-test sort
+    (c-should* (list 1 2 3) (list 1 2 3) #'<)
+    (c-should* (list 1 2 3) (list 3 2 1) #'<)
+    (c-should* '[1 2 3] '[1 2 3] #'<)
+    (c-should* '[1 2 3] '[3 2 1] #'<)))
+
+(ert-deftest compat-= ()
+  "Check if `=' was advised correctly."
+  (compat-test =
+    '(((0 0) t)
+      ((0 0 0) t)
+      ((0 0 0 0) t)
+      ((0 0 0 0 0) t)
+      ((0.0 0.0) t)
+      ((+0.0 -0.0) t)
+      ((0.0 0.0 0.0) t)
+      ((0.0 0.0 0.0 0.0) t)
+      ((0 1) nil)
+      ((0 0 1) nil)
+      ((0 0 0 0 1) nil)
+      ((0 0 a) nil wrong-type-argument)
+      ((0 1 a) nil)
+      ((0.0 0.0 0.0 0.1) nil)
+      )
+    (apply-partially #'c-= #'=)
+    #'=))
+
+(ert-deftest compat-< ()
+  "Check if `<' was advised correctly."
+  (compat-test <
+    (c-should* nil 0 0)
+    (c-should* nil 0 0 0)
+    (c-should* nil 0 0 0 0)
+    (c-should* nil 0 0 0 0 0)
+    (c-should* nil 0.0 0.0)
+    (c-should* nil +0.0 -0.0)
+    (c-should* nil 0.0 0.0 0.0)
+    (c-should* nil 0.0 0.0 0.0 0.0)
+    (c-should* t 0 1)
+    (c-should* nil 1 0)
+    (c-should* nil 0 0 1)
+    (c-should* t 0 1 2)
+    (c-should* nil 2 1 0)
+    (c-should* nil 0 0 0 0 1)
+    (c-should* t 0 1 2 3 4)
+    (c-error* wrong-type-argument 0 1 'a)
+    (c-should* nil 0 0 'a)
+    (c-should* nil 0.0 0.0 0.0 0.1)
+    (c-should* t -0.1 0.0 0.2 0.4)
+    (c-should* t -0.1 0 0.2 0.4)))
+
+(ert-deftest compat-> ()
+  "Check if `>' was advised correctly."
+  (compat-test >
+    (c-should* nil 0 0)
+    (c-should* nil 0 0 0)
+    (c-should* nil 0 0 0 0)
+    (c-should* nil 0 0 0 0 0)
+    (c-should* nil 0.0 0.0)
+    (c-should* nil +0.0 -0.0)
+    (c-should* nil 0.0 0.0 0.0)
+    (c-should* nil 0.0 0.0 0.0 0.0)
+    (c-should* t 1 0)
+    (c-should* nil 1 0 0)
+    (c-should* nil 0 1 2)
+    (c-should* t 2 1 0)
+    (c-should* nil 1 0 0 0 0)
+    (c-should* t 4 3 2 1 0)
+    (c-should* nil 4 3 2 1 1)
+    (c-error* wrong-type-argument 1 0 'a)
+    (c-should* nil 0 0 'a)
+    (c-should* nil 0.1 0.0 0.0 0.0)
+    (c-should* t 0.4 0.2 0.0 -0.1)
+    (c-should* t 0.4 0.2 0 -0.1)))
+
+(ert-deftest compat-<= ()
+  "Check if `<=' was advised correctly."
+  (compat-test <=
+    (c-should* t 0 0)
+    (c-should* t 0 0 0)
+    (c-should* t 0 0 0 0)
+    (c-should* t 0 0 0 0 0)
+    (c-should* t 0.0 0.0)
+    (c-should* t +0.0 -0.0)
+    (c-should* t 0.0 0.0 0.0)
+    (c-should* t 0.0 0.0 0.0 0.0)
+    (c-should* nil 1 0)
+    (c-should* nil 1 0 0)
+    (c-should* t 0 1 2)
+    (c-should* nil 2 1 0)
+    (c-should* nil 1 0 0 0 0)
+    (c-should* nil 4 3 2 1 0)
+    (c-should* nil 4 3 2 1 1)
+    (c-should* t 0 1 2 3 4)
+    (c-should* t 1 1 2 3 4)
+    (c-error* wrong-type-argument 0 0 'a)
+    (c-error* wrong-type-argument 0 1 'a)
+    (c-should* nil 1 0 'a)
+    (c-should* nil 0.1 0.0 0.0 0.0)
+    (c-should* t 0.0 0.0 0.0 0.1)
+    (c-should* t -0.1 0.0 0.2 0.4)
+    (c-should* t -0.1 0.0 0.0 0.2 0.4)
+    (c-should* t -0.1 0.0 0 0.2 0.4)
+    (c-should* t -0.1 0 0.2 0.4)
+    (c-should* nil 0.4 0.2 0.0 -0.1)
+    (c-should* nil 0.4 0.2 0.0 0.0 -0.1)
+    (c-should* nil 0.4 0.2 0 0.0 0.0 -0.1)
+    (c-should* nil 0.4 0.2 0 -0.1)))
+
+(ert-deftest compat->= ()
+  "Check if `>=' was implemented correctly."
+  (compat-test >=
+    (c-should* t 0 0)
+    (c-should* t 0 0 0)
+    (c-should* t 0 0 0 0)
+    (c-should* t 0 0 0 0 0)
+    (c-should* t 0.0 0.0)
+    (c-should* t +0.0 -0.0)
+    (c-should* t 0.0 0.0 0.0)
+    (c-should* t 0.0 0.0 0.0 0.0)
+    (c-should* t 1 0)
+    (c-should* t 1 0 0)
+    (c-should* nil 0 1 2)
+    (c-should* t 2 1 0)
+    (c-should* t 1 0 0 0 0)
+    (c-should* t 4 3 2 1 0)
+    (c-should* t 4 3 2 1 1)
+    (c-error* wrong-type-argument 0 0 'a)
+    (c-error* wrong-type-argument 1 0 'a)
+    (c-should* nil 0 1 'a)
+    (c-should* t 0.1 0.0 0.0 0.0)
+    (c-should* nil 0.0 0.0 0.0 0.1)
+    (c-should* nil -0.1 0.0 0.2 0.4)
+    (c-should* nil -0.1 0.0 0.0 0.2 0.4)
+    (c-should* nil -0.1 0.0 0 0.2 0.4)
+    (c-should* nil -0.1 0 0.2 0.4)
+    (c-should* t 0.4 0.2 0.0 -0.1)
+    (c-should* t 0.4 0.2 0.0 0.0 -0.1)
+    (c-should* t 0.4 0.2 0 0.0 0.0 -0.1)
+    (c-should* t 0.4 0.2 0 -0.1)))
+
+(ert-deftest compat-special-form-p ()
+  "Check if `special-form-p' was implemented correctly."
+  (compat-test special-form-p
+    (c-should t 'if)
+    (c-should t 'cond)
+    (c-should nil 'when)
+    (c-should nil 'defun)
+    (c-should nil '+)
+    (c-should nil nil)
+    (c-should nil "macro")
+    (c-should nil '(macro . +))))
+
+(ert-deftest compat-macrop ()
+  "Check if `macrop' was implemented correctly."
+  (compat-test macrop
+    (c-should t 'lambda)
+    (c-should t 'defun)
+    (c-should t 'defmacro)
+    (c-should nil 'defalias)
+    (c-should nil 'if)
+    (c-should nil '+)
+    (c-should nil 1)
+    (c-should nil nil)
+    (c-should nil "macro")
+    (c-should t '(macro . +))))
+
+(ert-deftest compat-string-suffix-p ()
+  "Check if `string-suffix-p' was implemented correctly."
+  (compat-test string-suffix-p
+    (c-should t "a" "abba")
+    (c-should t "ba" "abba")
+    (c-should t "abba" "abba")
+    (c-should nil "a" "ABBA")
+    (c-should nil "bA" "ABBA")
+    (c-should nil "aBBA" "ABBA")
+    (c-should nil "c" "ABBA")
+    (c-should nil "c" "abba")
+    (c-should nil "cddc" "abba")
+    (c-should nil "aabba" "abba")))
+
+(ert-deftest compat-split-string ()
+  "Check if `split-string' was advised correctly."
+  (compat-test split-string
+    (c-should* '("a" "b" "c") "a b c")
+    (c-should* '("..a.." "..b.." "..c..") "..a.. ..b.. ..c..")
+    (c-should* '("a" "b" "c") "..a.. ..b.. ..c.." nil nil "\\.+")))
+
+(ert-deftest compat-delete-consecutive-dups ()
+  "Check if `delete-consecutive-dups' was implemented correctly."
+  (compat-test delete-consecutive-dups
+    (c-should '(1 2 3 4) '(1 2 3 4))
+    (c-should '(1 2 3 4) '(1 2 2 3 4 4))
+    (c-should '(1 2 3 2 4) '(1 2 2 3 2 4 4))))
+
+(ert-deftest compat-autoloadp ()
+  "Check if `autoloadp' was implemented correctly."
+  (compat-test autoloadp
+    (c-should t '(autoload . anything))
+    (c-should nil 'anything)))
+
+(ert-deftest compat-file-name-base ()
+  "Check if `file-name-base' was implemented correctly."
+  (compat-test file-name-base
+    (c-should "file" "file.txt")
+    (c-should "file" "/path/to/some/file.txt")
+    (c-should "file" "/path/to/some/file")))
+
+(ert-deftest compat-posnp ()
+  "Check if `posnp' was implemented correctly."
+  (compat-test posnp
+    ;; FIXME: return an actual posn.
+    ;; (c-should t (posn-at-point))
+    (c-should nil (current-buffer))
+    (c-should nil (point-max))
+    (c-should nil (point-min))
+    (c-should nil nil)))
+
+(ert-deftest compat-string-clean-whitespace ()
+  "Check if `string-clean-whitespace' was implemented correctly."
+  (compat-test string-clean-whitespace
+    (c-should "a b c" "a b c")
+    (c-should "a b c" "   a b c")
+    (c-should "a b c" "a b c   ")
+    (c-should "a b c" "a    b c")
+    (c-should "a b c" "a b    c")
+    (c-should "a b c" "a    b    c")
+    (c-should "a b c" "   a    b    c")
+    (c-should "a b c" "a    b    c    ")
+    (c-should "a b c" "   a    b    c    ")
+    (c-should "aa bb cc" "aa bb cc")
+    (c-should "aa bb cc" "   aa bb cc")
+    (c-should "aa bb cc" "aa bb cc   ")
+    (c-should "aa bb cc" "aa    bb cc")
+    (c-should "aa bb cc" "aa bb    cc")
+    (c-should "aa bb cc" "aa    bb    cc")
+    (c-should "aa bb cc" "   aa    bb    cc")
+    (c-should "aa bb cc" "aa    bb    cc    ")
+    (c-should "aa bb cc" "   aa    bb    cc    ")))
+
+(ert-deftest compat-string-fill ()
+  "Check if `string-fill' was implemented correctly."
+  (compat-test string-fill
+    (c-should "a a a a a" "a a a a a" 9)
+    (c-should "a a a a a" "a a a a a" 10)
+    (c-should "a a a a\na" "a a a a a" 8)
+    (c-should "a a a a\na" "a  a  a  a  a" 8)
+    (c-should "a a\na a\na" "a a a a a" 4)
+    (c-should "a\na\na\na\na" "a a a a a" 2)
+    (c-should "a\na\na\na\na" "a a a a a" 1)))
+
+(ert-deftest compat-string-lines ()
+  "Check if `string-lines' was implemented correctly."
+  (compat-test string-lines
+    (c-should '("a" "b" "c") "a\nb\nc")
+    (c-should '("a" "b" "c" "") "a\nb\nc\n")
+    (c-should '("a" "b" "c") "a\nb\nc\n" t)
+    (c-should '("abc" "bcd" "cde") "abc\nbcd\ncde")
+    (c-should '(" abc" " bcd " "cde ") " abc\n bcd \ncde ")))
+
+(ert-deftest compat-string-pad ()
+  "Check if `string-pad' was implemented correctly."
+  (compat-test string-pad
+    (c-should "a   " "a" 4)
+    (c-should "aaaa" "aaaa" 4)
+    (c-should "aaaaaa" "aaaaaa" 4)
+    (c-should "a..." "a" 4 ?.)
+    (c-should "   a" "a" 4 nil t)
+    (c-should "...a" "a" 4 ?. t)))
+
+(ert-deftest compat-string-chop-newline ()
+  "Check if `string-chop-newline' was implemented correctly."
+  (compat-test string-chop-newline
+    (c-should "" "")
+    (c-should "" "\n")
+    (c-should "aaa" "aaa")
+    (c-should "aaa" "aaa\n")
+    (c-should "aaa\n" "aaa\n\n")))
+
+(ert-deftest compat-macroexpand-1 ()
+  "Check if `macroexpand-1' was implemented correctly."
+  (compat-test macroexpand-1
+    (c-should '(if a b c) '(if a b c))
+    (c-should '(if a (progn b)) '(when a b))
+    (c-should '(if a (progn (unless b c))) '(when a (unless b c)))))
+
+(ert-deftest compat-file-size-human-readable ()
+  "Check if `file-size-human-readable' was advised properly."
+  (compat-test file-size-human-readable
+    (c-should* "1000" 1000)
+    (c-should* "1k" 1024)
+    (c-should* "1M" (* 1024 1024))
+    (c-should* "1G" (expt 1024 3))
+    (c-should* "1T" (expt 1024 4))
+    (c-should* "1k" 1000 'si)
+    (c-should* "1KiB" 1024 'iec)
+    (c-should* "1KiB" 1024 'iec)
+    (c-should* "1 KiB" 1024 'iec " ")
+    (c-should* "1KiA" 1024 'iec nil "A")
+    (c-should* "1 KiA" 1024 'iec " " "A")
+    (c-should* "1kA" 1000 'si nil "A")
+    (c-should* "1 k" 1000 'si " ")
+    (c-should* "1 kA" 1000 'si " " "A")))
+
+(ert-deftest compat-format-prompt ()
+  "Check if `file-size-human-readable' was implemented properly."
+  (compat-test format-prompt
+    (c-should "Prompt: " "Prompt" nil)
+    (c-should "Prompt (default 3): " "Prompt" 3)
+    (c-should "Prompt (default abc): " "Prompt" "abc")
+    (c-should "Prompt (default abc def): " "Prompt" "abc def")
+    (c-should "Prompt 10: " "Prompt %d" nil 10)
+    (c-should "Prompt \"abc\" (default 3): " "Prompt %S" 3 "abc")))
+
+(provide 'compat-tests)
+;;; compat-tests.el ends here
+
+;; Local Variables:
+;; elisp-shorthands: (("c-" . "compat--"))
+;; End:
diff --git a/compat.el b/compat.el
new file mode 100644
index 0000000..c9b9103
--- /dev/null
+++ b/compat.el
@@ -0,0 +1,90 @@
+;;; compat.el --- Compatibility Library              -*- lexical-binding: t; 
-*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Author: Philip Kaludercic <philipk@posteo.net>
+;; Maintainer: Philip Kaludercic <philipk@posteo.net>
+;; Version: 28.1.0.0-dev
+;; URL: https://git.sr.ht/~pkal/compat/
+;; Package-Requires: ((emacs "24.1"))
+;; Keywords: lisp
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; To allow for the usage of Emacs functions and macros that are
+;; defined in newer versions of Emacs, compat.el provides definitions
+;; that are installed ONLY if necessary.  These reimplementations of
+;; functions and macros are at least subsets of the actual
+;; implementations.  Be sure to read the documentation string to make
+;; sure.
+;;
+;; Not every function provided in newer versions of Emacs is provided
+;; here.  Some depend on new features from the core, others cannot be
+;; implemented to a meaningful degree.  The main audience for this
+;; library are not regular users, but package maintainers.  Therefore
+;; commands and user options are usually not implemented here.
+
+;;; Code:
+
+(eval-when-compile (require 'compat-macs))
+
+;;;; Core functionality
+
+;; Suppress errors triggered by requiring non-existent libraries in
+;; older versions of Emacs (e.g. subr-x).
+(compat-advise require (feature &optional filename noerror)
+  "Avoid throwing an error if library has compatibility code."
+  :force t
+  (condition-case err
+      (funcall oldfun feature filename noerror)
+    (file-missing
+     ;; FIXME: avoid false negatives, check if compat defined a
+     ;;        feature.
+     (unless (assq feature after-load-alist)
+       (signal (car err) (cdr err))))))
+
+;; Load the actual compatibility definitions:
+(when (version< emacs-version "24.2")
+  (require 'compat-24.2))
+(when (version< emacs-version "24.3")
+  (require 'compat-24.3))
+(when (version< emacs-version "24.4")
+  (require 'compat-24.4))
+(when (version< emacs-version "25")
+  (require 'compat-25.1))
+(when (version< emacs-version "26")
+  (require 'compat-26.1))
+(when (version< emacs-version "27")
+  (require 'compat-27.1))
+(when (version< emacs-version "28")
+  (require 'compat-28.1))
+
+;;;; Etcetera
+
+;; ;;;; Ensure NonGNU ELPA is in `package-archives'
+;; (with-eval-after-load 'package
+;;   (unless (or (rassoc "https://elpa.nongnu.org/nongnu/"; package-archives)
+;;            (rassoc "http://elpa.nongnu.org/nongnu/"; package-archives)
+;;            (rassoc "https://elpa.nongnu.org/nongnu-devel/"; package-archives)
+;;            (rassoc "http://elpa.nongnu.org/nongnu-devel/"; package-archives))
+;;     (add-to-list 'package-archives '("nongnu" . 
"https://elpa.nongnu.org/nongnu/";))))
+
+;; To ensure that compat.el is loaded as soon as possible, a require
+;; call is inserted directly into the autoload file:
+;;;###autoload (require 'compat)
+
+(provide 'compat)
+;;; compat.el ends here



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