File: groff.info,  Node: Dummy Characters,  Prev: Italic Corrections,  Up: Using Fonts

5.19.10 Dummy Characters
------------------------

As discussed in *note Requests and Macros::, the first character on an
input line is treated specially.  Further, formatting a glyph has many
consequences on formatter state (*note Environments::).  Occasionally,
we want to escape this context or embrace some of those consequences
without actually rendering a glyph to the output.

 -- Escape sequence: \&
     Interpolate a dummy character, which is constitutive of output but
     invisible.(1)  (*note Dummy Characters-Footnote-1::) Its presence
     alters the interpretation context of a subsequent input character,
     and enjoys several applications.

        * Prevent insertion of extra space after an end-of-sentence
          character.

               Test.
               Test.
                   => Test.  Test.
               Test.\&
               Test.
                   => Test. Test.

        * Prevent recognition of a control character.

               .Test
                   error-> warning: macro 'Test' not defined
               \&.Test
                   => .Test

        * Prevent kerning between two glyphs.

        * Translate a character to "nothing".

               .tr JIjiK\&k\&UVuv
               Post universitum, alea jacta est, OK?
                   => Post vniversitvm, alea iacta est, O?

     The dummy character escape sequence sees use in macro definitions
     as a means of ensuring that arguments are treated as text even if
     they begin with spaces or control characters.

          .de HD \" typeset a simple bold heading
          .  sp
          .  ft B
          \&\\$1 \" exercise: remove the \&
          .  ft
          .  sp
          ..
          .HD .\|.\|.\|surprised?

   One way to think about the dummy character is to imagine placing the
symbol '&' in the input at a certain location; if doing so has all the
side effects on formatting that you desire except for sticking an ugly
ampersand in the midst of your text, the dummy character is what you
want in its place.

 -- Escape sequence: \)
     Interpolate a transparent dummy character--one that is transparent
     to end-of-sentence detection.  It behaves as '\&', except that '\&'
     is treated as letters and numerals normally are after '.', '?' and
     '!'; '\&' cancels end-of-sentence detection, and '\)' does not.

          .de Suffix-&
          .  nop \&\\$1
          ..
          .
          .de Suffix-)
          .  nop \)\\$1
          ..
          .
          Here's a sentence.\c
          .Suffix-& '
          Another one.\c
          .Suffix-) '
          And a third.
              => Here's a sentence.' Another one.'  And a third.