texinfo: @set @value

@set @clear @value

@ifset @ifclear

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18.5.1 '@set' and '@value'
--------------------------

You use the '@set' command to specify a value for a flag, which is later
expanded by the '@value' command.

  A "flag" (aka "variable") name is an identifier starting with an
alphanumeric, '-', or '_'.  Subsequent characters, if any, may not be
whitespace, '@', braces, angle brackets, or any of '~`^+|'; other
characters, such as '%', may work.  However, it is best to use only
letters and numerals in a flag name, not '-' or '_' or others--they will
work in some contexts, but not all, due to limitations in TeX.

  The value is the remainder of the input line, and can contain
anything.  However, unlike most other commands which take the rest of
the line as a value, '@set' need not appear at the beginning of a line.

  Write the '@set' command like this:

     @set foo This is a string.

This sets the value of the flag 'foo' to "This is a string.".

  The Texinfo formatters then replace an '@value{FLAG}' command with the
string to which FLAG is set.  Thus, when 'foo' is set as shown above,
the Texinfo formatters convert this:

     @value{foo}
to this:
     This is a string.

  You can write an '@value' command within a paragraph; but you must
write an '@set' command on a line of its own.

  If you write the '@set' command like this:

     @set foo

without specifying a string, the value of 'foo' is the empty string.

  If you clear a previously set flag with '@clear FLAG', a subsequent
'@value{flag}' command will report an error.

  For example, if you set 'foo' as follows:

     @set howmuch very, very, very

then the formatters transform

     It is a @value{howmuch} wet day.
into
     It is a very, very, very wet day.

  If you write

     @clear howmuch

then the formatters transform

     It is a @value{howmuch} wet day.
into
     It is a {No value for "howmuch"} wet day.

  '@value' cannot be reliably used as the argument to an accent command
(Note: Inserting Accents).  For example, this fails:

     @set myletter a
     @'@value{myletter}