[docs]: Tweak escape/control character discussion.

* Introduce term "ordinary character", an input character that is not
  invalid (cf. "special character").
* Recast discussion of control, no-break control, and escape control
  characters in terms of ordinary characters, ruling out the surmise
  that a special character can serve as any of these.
* Parallelize wording of these.
* Tighten and align wording with this terminology.

2 files changed, 67 insertions, 64 deletions

diff --git a/doc/groff.texi b/doc/groff.texi
index b0ef7a91e..abedde37f 100644
--- a/doc/groff.texi
+++ b/doc/groff.texi
@@ -6360,12 +6360,14 @@ not qualify, so our first attempt got a warning.
 @section Identifiers
 @cindex identifiers
 
-GNU @code{troff} has rules for properly formed
-@dfn{identifiers}---labels for objects with syntactical importance, like
-registers, names (macros, strings, or diversions), typefaces, glyphs,
-colors, character classes, environments, and streams.  An identifier
-consists of one or more characters excepting spaces, tabs, newlines, and
-invalid input characters.
+An @dfn{identifier} is a label for an object of syntactical
+importance:@: a register, name (macro, string, or diversion), typeface,
+color, special character, character class, environment, or stream.
+Valid identifiers consist of one or more ordinary characters.
+@cindex ordinary character
+@cindex character, ordinary
+An @slanted{ordinary character} is an input character that is not a
+leader, tab, newline, or invalid as GNU @code{troff} input.
 
 @c XXX: We might move this discussion earlier since it is applicable to
 @c troff input in general, and include a reference to the `trin`
@@ -6403,11 +6405,10 @@ these ranges.@footnote{Consider what happens when a C1 control
 @code{0x80}--@code{0x9F} is necessary as a continuation byte in a UTF-8
 sequence.}
 
-The identifiers @samp{br}, @samp{PP}, @samp{end-list},
+Thus, the identifiers @samp{br}, @samp{PP}, @samp{end-list},
 @samp{ref*normal-print}, @samp{|}, @samp{@@_}, and @samp{!"#$%'()*+,-./}
 are all valid.  Discretion should be exercised to prevent confusion.
-Some care is required with identifiers starting with @samp{(} or
-@samp{[}.
+Identifiers starting with @samp{(} or @samp{[} require care.
 
 @Example
 .nr x 9
@@ -6585,24 +6586,25 @@ invoke requests that don't cause breaks is harmless but poor style.
 @cindex no-break control character, changing (@code{c2})
 @cindex character, no-break control, changing (@code{c2})
 @cindex control character, no-break, changing (@code{c2})
-The control character @samp{.} and the no-break control character
-@samp{'} can be changed with the @code{cc} and @code{c2} requests,
-respectively.
+The control @samp{.} and no-break control @samp{'} characters can each
+be changed to any ordinary character@footnote{Recall @ref{Identifiers}.}
+with the @code{cc} and @code{c2} requests, respectively.
+
+@Defreq {cc, [@Var{o}]}
+Recognize the ordinary character@tie{}@var{o} as the control character.
+If@tie{}@var{o} is absent or invalid, the default control character
+@samp{.} is selected.  The identity of the control character is
+associated with the environment (@pxref{Environments}).
+@endDefreq
 
-@Defreq {cc, [@Var{c}]}
-Set the control character to@tie{}@var{c}.  With no argument, the
-default control character @samp{.} is restored.  The identity of the
+@Defreq {c2, [@Var{o}]}
+Recognize the ordinary character@tie{}@var{o} as the no-break control
+character.  If@tie{}@var{o} is absent or invalid, the default no-break
+control character @samp{'} is selected.  The identity of the no-break
 control character is associated with the environment
 (@pxref{Environments}).
 @endDefreq
 
-@Defreq {c2, [@Var{c}]}
-Set the no-break control character to@tie{}@var{c}.  With no argument,
-the default no-break control character @samp{'} is restored.  The
-identity of the no-break control character is associated with the
-environment (@pxref{Environments}).
-@endDefreq
-
 When writing a macro, you might wish to know which control character was
 used to call it.
 
@@ -6949,11 +6951,12 @@ invoked, no input character is recognized as starting an escape
 sequence in interpretation mode.
 @endDefreq
 
-@Defreq {ec, [@Var{c}]}
+@Defreq {ec, [@Var{o}]}
 @cindex escape character, changing (@code{ec})
 @cindex character, escape, changing (@code{ec})
-Recognize@tie{}@var{c} as the escape character.  If@tie{}@var{c} is
-absent or invalid, the default escape character @samp{\} is selected.
+Recognize the ordinary character@tie{}@var{o} as the escape character.
+If@tie{}@var{o} is absent or invalid, the default escape character
+@samp{\} is selected.
 
 Changing the escape character globally likely breaks macro packages,
 since GNU @code{troff} has no mechanism to ``intern'' macros, that is,
@@ -10742,8 +10745,8 @@ are translated on input to the special character escape sequences
 @code{\[aa]}, @code{\[-]}, @code{\[ul]}, and @code{\[ga]}, respectively.
 
 A special character name of length one is not the same thing as an
-ordinary input character: that is, the character @code{a} is not the
-same as @code{\[a]}.
+ordinary character: that is, the character @code{a} is not the same as
+@code{\[a]}.
 
 If @var{name} is undefined, a warning in category @samp{char} is
 produced and the escape is ignored.  @xref{Warnings}, for information
@@ -12533,13 +12536,10 @@ termed @dfn{conditional expressions}.
 
 @table @code
 @item c @var{glyph}
-True if @var{glyph} is available, where @var{glyph} is
-@c TODO: "an ordinary character", after we update the section "Fonts and
-@c Symbols" to better parallelize it with groff_char(7).
-a Unicode basic Latin character, a GNU @code{troff} special character
-@samp{\(@var{xx}} or @samp{\[@var{xxx}]}, @samp{\N'@var{xxx}'}, or has
-been defined by any of the @code{char}, @code{fchar}, @code{fschar}, or
-@code{schar} requests.
+True if @var{glyph} is available, where @var{glyph} is an ordinary
+character, a special character @samp{\(@var{xx}} or @samp{\[@var{xxx}]},
+@samp{\N'@var{xxx}'}, or has been defined by any of the @code{char},
+@code{fchar}, @code{fschar}, or @code{schar} requests.
 
 @item d @var{name}
 True if a string, macro, diversion, or request called @var{name} exists.
diff --git a/man/groff.7.man b/man/groff.7.man
index 8c50052bc..07944567d 100644
--- a/man/groff.7.man
+++ b/man/groff.7.man
@@ -1254,28 +1254,29 @@ containing them is surrounded by parentheses.
 .\" ====================================================================
 .
 .\" BEGIN Keep (roughly) parallel with groff.texi node "Identifiers".
-GNU
+An identifier is a label for an object of syntactical importance:
+a register,
+name
+(macro,
+string,
+or diversion),
+typeface,
+color,
+special character,
+character class,
+environment,
+or stream.
+.
+Valid identifiers consist of one or more ordinary characters.
+.
+An
+.I ordinary character
+is an input character that is not a leader,
+tab,
+newline,
+or invalid as GNU
 .I troff \" GNU
-has rules for properly formed
-.IR identifiers \[em]labels
-for objects with syntactical importance,
-like registers,
-names
-(macros,
-strings,
-or diversions),
-typefaces,
-glyphs,
-colors,
-character classes,
-environments,
-and streams.
-.
-An identifier consists of one or more characters excepting
-spaces,
-tabs,
-newlines,
-and invalid input characters.
+input.
 .
 .
 .\" XXX: We might move this discussion earlier since it is applicable to
@@ -2577,9 +2578,10 @@ Reset no-break control character to
 .dquoted_char \[aq] .
 .
 .TPx
-.REQ .c2 "c"
-Set no-break control character to
-.IR c .
+.REQ .c2 "o"
+Recognize ordinary character
+.I o
+as the no-break control character.
 .
 .TPx
 .REQ .cc
@@ -2587,9 +2589,10 @@ Reset control character to
 .squoted_char . .
 .
 .TPx
-.REQ .cc "c"
-Set control character to
-.IR c .
+.REQ .cc "o"
+Recognize ordinary character
+.I o
+as the control character.
 .
 .TPx
 .REQ .ce
@@ -2951,9 +2954,9 @@ as the escape character.
 .
 .
 .TPx
-.REQ .ec "c"
-Recognize
-.I c
+.REQ .ec "o"
+Recognize ordinary character
+.I o
 as the escape character.
 .
 .