[groff] 04/06: [docs]: Clarify vertical position trap material.

[G. Branden Robinson]

gbranden pushed a commit to branch master
in repository groff.

commit ad0a8212dfa2f439b6ab910ff7233d2000944604
Author: G. Branden Robinson <g.branden.robinson@gmail.com>
AuthorDate: Sat Jun 24 17:31:18 2023 -0500

    [docs]: Clarify vertical position trap material.
    
    * Move sentence about how these are sprung to correctly generalize.
    * Emphasize contrast between page location traps (many per page) and
      diversion traps (one per diversion).
---
 doc/groff.texi  | 22 +++++++++++++---------
 man/groff.7.man | 47 ++++++++++++++++++++++++++++++++++++++---------
 2 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/doc/groff.texi b/doc/groff.texi
index a9961978e..2a6635e9d 100644
--- a/doc/groff.texi
+++ b/doc/groff.texi
@@ -14347,10 +14347,11 @@ It is said that a trap is @dfn{sprung} if its 
condition is fulfilled.
 @cindex vertical position traps
 @cindex traps, vertical position
 
-@dfn{Vertical position traps} perform an action when GNU @code{troff}
-reaches or passes a certain vertical location on the output page or in a
-diversion.  Their applications include setting page headers and footers,
-body text in multiple columns, and footnotes.
+A @dfn{vertical position trap} calls a macro when the formatter's
+vertical drawing position reaches or passes, in the downward direction,
+a certain location on the output page or in a diversion.  Its
+applications include setting page headers and footers, body text in
+multiple columns, and footnotes.
 
 @DefreqList {vpt, [@Var{flag}]}
 @DefregListEndx {.vpt}
@@ -14385,9 +14386,9 @@ Implicit Page Trap}.
 @cindex page location traps
 @cindex traps, page location
 
-A @dfn{page location trap} is sprung when the vertical drawing position
-reaches or passes, in the downward direction, the place at which it is
-planted.
+A @dfn{page location trap} is a vertical position trap that applies to
+the page; that is, to undiverted output.  Many can be present; manage
+them with the @code{wh} and @code{ch} requests.
 
 @Defreq {wh, dist [@Var{name}]}
 Plant macro @var{name} as page location trap at @var{dist}.  The default
@@ -14686,14 +14687,17 @@ are disabled with GNU @code{troff}'s @code{vpt} 
request.
 @cindex diversion traps
 @cindex traps, diversion
 
+A diversion is not formatted in the context of a page, so it lacks page
+location traps; instead it can have a @dfn{diversion trap}.  There can
+exist at most one such vertical position trap per diversion.
+
 @Defreq {dt, [@Var{dist} @Var{name}]}
 @cindex @code{.t} register, and diversions
 @cindex setting diversion trap (@code{dt})
 @cindex diversion trap, setting (@code{dt})
 @cindex trap, diversion, setting (@code{dt})
 Set a trap @emph{within} a diversion at location @var{dist}, which is
-interpreted relative to diversion rather than page boundaries.  There
-exists only a single diversion trap per diversion.  If invoked with
+interpreted relative to diversion rather than page boundaries.  If invoked with
 fewer than two arguments, any diversion trap in the current diversion is
 removed.  The register @code{.t} works within diversions.  It is an
 error to invoke @code{dt} in the top-level diversion.
diff --git a/man/groff.7.man b/man/groff.7.man
index 71758cb24..566737f21 100644
--- a/man/groff.7.man
+++ b/man/groff.7.man
@@ -7189,6 +7189,17 @@ or conditions on the input that,
 when reached or fulfilled,
 call a specified macro.
 .
+A
+.I "vertical position trap"
+calls a macro when the formatter's vertical drawing position reaches or
+passes,
+in the downward direction,
+a certain location on the output page or in a diversion.
+.
+Its applications include setting page headers and footers,
+body text in multiple columns,
+and footnotes.
+.
 These traps can occur at a given location on the page
 .RB ( .wh ,\~ .ch );
 at a given location in the current diversion
@@ -7196,7 +7207,20 @@ at a given location in the current diversion
 these are known as
 vertical position traps,
 which can be disabled and re-enabled
-.RB ( .vpt );
+.RB ( .vpt ).
+.
+.
+.P
+A diversion is not formatted in the context of a page,
+so it lacks page location traps;
+instead it can have a
+.I "diversion trap."
+.
+There can exist at most one such vertical position trap per diversion.
+.
+.
+.P
+Other kinds of trap can be planted
 at a blank line
 .RB ( .blm );
 at a line with leading space characters
@@ -7246,14 +7270,19 @@ with its corresponding amount of motion
 .\" ====================================================================
 .
 A
-.I page location trap
-is sprung when the vertical drawing position reaches or passes,
-in the downward direction,
-the place at which it is planted
-(with
-.B .wh
-or
-.BR .ch ).
+.I "page location trap"
+is a vertical position trap that applies to
+the page;
+that is,
+to undiverted output.
+.
+Many can be present;
+manage them with
+the
+.B wh
+and
+.B ch
+requests.
 .
 Non-negative page locations given to these requests set the trap
 relative to the top of the page;