cex: update NEWS for 3.7

[Akim Demaille]

commit 30ba40ba4c46b7b1824707679bafc0af6acd556a
Author: Akim Demaille <akim.demaille@gmail.com>
Date:   Tue Jul 21 07:41:06 2020 +0200

    cex: update NEWS for 3.7
    
    * NEWS: Update to the current style of cex display.

diff --git a/NEWS b/NEWS
index deca6100..d42ada62 100644
--- a/NEWS
+++ b/NEWS
@@ -2,39 +2,6 @@ GNU Bison NEWS
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
-
-* Noteworthy changes in release 3.6.93 (2020-07-20) [beta]
-
-** Bug fixes
-
-  Portability issues.
-
-
-* Noteworthy changes in release 3.6.92 (2020-07-19) [beta]
-
-Changes in the display of counterexamples.
-
-** Documentation
-
-*** Examples
-
-  The bistromathic demonstrates %param and how to quote sources in the error
-  messages:
-
-    > 123 456
-    1.5-7: syntax error: expected end of file or + or - or * or / or ^ before 
number
-        1 | 123 456
-          |     ^~~
-
-* Noteworthy changes in release 3.6.91 (2020-07-09) [beta]
-
-** Bug fixes
-
-  Portability issues.
-
-
-* Noteworthy changes in release 3.6.90 (2020-07-04) [beta]
-
 ** Deprecated features
 
   The YYPRINT macro, which works only with yacc.c and only for tokens, was
@@ -51,53 +18,115 @@ Changes in the display of counterexamples.
 
   Contributed by Vincent Imbimbo.
 
-  When given `--report=counterexamples` or `-Wcounterexamples`, bison will
-  now output counterexamples for conflicts in the grammar.  These are
-  strings in the grammar which can be parsed in two ways due to the
-  conflict.  For example:
+  When given `-Wcounterexamples`/`-Wcex`, bison will now output
+  counterexamples for conflicts in the grammar.
 
-    Shift/reduce conflict on token "/":
-      Example              exp "+" exp • "/" exp
+**** Unifying Counterexamples
+
+  Unifying counterexamples are strings which can be parsed in two ways due
+  to the conflict.  For example on a grammar that contains the usual
+  "dangling else" ambiguity:
+
+    $ bison else.y
+    else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+    else.y: note: rerun with option '-Wcounterexamples' to generate conflict 
counterexamples
+
+    $ bison else.y -Wcex
+    else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+    else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
+      Example: "if" exp "then" "if" exp "then" exp • "else" exp
       Shift derivation
         exp
-        ↳ exp "+" exp
-                  ↳ exp • "/" exp
-      Example              exp "+" exp • "/" exp
+        ↳ "if" exp "then" exp
+                          ↳ "if" exp "then" exp • "else" exp
+      Example: "if" exp "then" "if" exp "then" exp • "else" exp
       Reduce derivation
         exp
-        ↳ exp             "/" exp
-          ↳ exp "+" exp •
-
-  When Bison is installed with text styling enabled, the example is actually
-  shown twice, with colors highlighting the ambiguity.
-
-  This is a shift/reduce conflict caused by none of the operators having
-  precedence, so the example can be parsed in the two ways shown.  When
-  bison cannot find an example that can be derived in two ways, it instead
-  generates two examples that are the same up until the dot:
-
-    First example        expr • ID ',' ID $end
-    Shift derivation
-      $accept
-      ↳ s                      $end
-        ↳ a                 ID
-          ↳ expr
-            ↳ expr • ID ','
-    Second example       expr • ID $end
-    Reduce derivation
-      $accept
-      ↳ s             $end
-        ↳ a        ID
-          ↳ expr •
+        ↳ "if" exp "then" exp                     "else" exp
+                          ↳ "if" exp "then" exp •
 
-  In these cases, the parser usually doesn't have enough lookahead to
-  differentiate the two given examples.
+  When text styling is enabled, colors are used in the examples and the
+  derivations to highlight the structure of both analyses.  In this case,
+
+    "if" exp "then" [ "if" exp "then" exp • ] "else" exp
+
+  vs.
+
+    "if" exp "then" [ "if" exp "then" exp • "else" exp ]
 
-  The counterexamples are "focused" in two different ways.  First, they do
+
+  The counterexamples are "focused", in two different ways.  First, they do
   not clutter the output with all the derivations from the start symbol,
   rather they start on the "conflicted nonterminal". They go straight to the
   point.  Second, they don't "expand" nonterminal symbols uselessly.
 
+**** Nonunifying Counterexamples
+
+  In the case of the dangling else, Bison found an example that can be
+  parsed in two ways (therefore proving that the grammar is ambiguous).
+  When it cannot find such an example, it instead generates two examples
+  that are the same up until the dot:
+
+    $ bison foo.y
+    foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+    foo.y: note: rerun with option '-Wcounterexamples' to generate conflict 
counterexamples
+    foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
+        4 | a: expr
+          |    ^~~~
+
+    $ bison -Wcex foo.y
+    foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+    foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
+      First example: expr • ID ',' ID $end
+      Shift derivation
+        $accept
+        ↳ s                      $end
+          ↳ a                 ID
+            ↳ expr
+              ↳ expr • ID ','
+      Second example: expr • ID $end
+      Reduce derivation
+        $accept
+        ↳ s             $end
+          ↳ a        ID
+            ↳ expr •
+    foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
+        4 | a: expr
+          |    ^~~~
+
+  In these cases, the parser usually doesn't have enough lookahead to
+  differentiate the two given examples.
+
+**** Reports
+
+  Counterexamples are also included in the report when given
+  `--report=counterexamples`/`-rcex` (or `--report=all`), with more
+  technical details:
+
+    State 7
+
+      1 exp: "if" exp "then" exp •  [$end, "then", "else"]
+      2    | "if" exp "then" exp • "else" exp
+
+      "else"  shift, and go to state 8
+
+      "else"    [reduce using rule 1 (exp)]
+      $default  reduce using rule 1 (exp)
+
+      shift/reduce conflict on token "else":
+          1 exp: "if" exp "then" exp •
+          2 exp: "if" exp "then" exp • "else" exp
+        Example: "if" exp "then" "if" exp "then" exp • "else" exp
+        Shift derivation
+          exp
+          ↳ "if" exp "then" exp
+                            ↳ "if" exp "then" exp • "else" exp
+        Example: "if" exp "then" "if" exp "then" exp • "else" exp
+        Reduce derivation
+          exp
+          ↳ "if" exp "then" exp                     "else" exp
+                            ↳ "if" exp "then" exp •
+
 *** File prefix mapping
 
   Contributed by Joshua Watt.
@@ -110,6 +139,11 @@ Changes in the display of counterexamples.
 
 ** Changes
 
+*** Diagnostics
+
+  When text styling is enabled and the terminal supports it, the warnings
+  now include hyperlinks to the documentation.
+
 *** Relocatable installation
 
   When installed to be relocatable (via `configure --enable-relocatable`),
@@ -150,6 +184,18 @@ Changes in the display of counterexamples.
   Now the parser state can be examined when parsing is finished.  The parser
   state is reset when starting a new parse.
 
+** Documentation
+
+*** Examples
+
+  The bistromathic demonstrates %param and how to quote sources in the error
+  messages:
+
+    > 123 456
+    1.5-7: syntax error: expected end of file or + or - or * or / or ^ before 
number
+        1 | 123 456
+          |     ^~~
+
 ** Bug fixes
 
 *** Include the generated header (yacc.c)
@@ -4415,7 +4461,7 @@ LocalWords:  yysymbol yytnamerr yyreport ctx ARGMAX 
yysyntax stderr LPAREN
 LocalWords:  symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
 LocalWords:  autocompletion bistromathic submessages Cayuela lexcalc hoc
 LocalWords:  yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
-LocalWords:  gotos readline Imbimbo Wcounterexamples
+LocalWords:  gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
 
 Local Variables:
 ispell-dictionary: "american"
diff --git a/src/derivation.c b/src/derivation.c
index 1e0ff4b7..87863527 100644
--- a/src/derivation.c
+++ b/src/derivation.c
@@ -193,9 +193,8 @@ all_spaces (const char *s)
 //
 // This function implements this.
 //
-// When COND is true, put S on OUT, preceeded by *PADDING white
-// spaces.  Otherwise add the width to *PADDING.  Return the width of
-// S.
+// When COND is true, put S on OUT, preceded by *PADDING white spaces.
+// Otherwise add the width to *PADDING.  Return the width of S.
 static int
 fputs_if (bool cond, FILE *out, int *padding, const char *s)
 {