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 5.2 If-else construct, or multibranch
 =====================================
 
 The other conditional, `ifelse', is much more powerful.  It can be used
 as a way to introduce a long comment, as an if-else construct, or as a
 multibranch, depending on the number of arguments supplied:
 
  -- Builtin: ifelse (COMMENT)
  -- Builtin: ifelse (STRING-1, STRING-2, EQUAL, [NOT-EQUAL])
  -- Builtin: ifelse (STRING-1, STRING-2, EQUAL-1, STRING-3, STRING-4,
           EQUAL-2, ...)
      Used with only one argument, the `ifelse' simply discards it and
      produces no output.
 
      If called with three or four arguments, `ifelse' expands into
      EQUAL, if STRING-1 and STRING-2 are equal (character for
      character), otherwise it expands to NOT-EQUAL.  A final fifth
      argument is ignored, after triggering a warning.
 
      If called with six or more arguments, and STRING-1 and STRING-2
      are equal, `ifelse' expands into EQUAL-1, otherwise the first
      three arguments are discarded and the processing starts again.
 
      The macro `ifelse' is recognized only with parameters.
 
    Using only one argument is a common `m4' idiom for introducing a
 block comment, as an alternative to repeatedly using `dnl'.  This
 special usage is recognized by GNU `m4', so that in this case, the
 warning about missing arguments is never triggered.
 
      ifelse(`some comments')
      =>
      ifelse(`foo', `bar')
      error-->m4:stdin:2: Warning: too few arguments to builtin `ifelse'
      =>
 
    Using three or four arguments provides decision points.
 
      ifelse(`foo', `bar', `true')
      =>
      ifelse(`foo', `foo', `true')
      =>true
      define(`foo', `bar')
      =>
      ifelse(foo, `bar', `true', `false')
      =>true
      ifelse(foo, `foo', `true', `false')
      =>false
 
    Notice how the first argument was used unquoted; it is common to
 compare the expansion of a macro with a string.  With this macro, you
 can now reproduce the behavior of many of the builtins, where the macro
 is recognized only with arguments.
 
      define(`foo', `ifelse(`$#', `0', ``$0'', `arguments:$#')')
      =>
      foo
      =>foo
      foo()
      =>arguments:1
      foo(`a', `b', `c')
      =>arguments:3
 
    However, `ifelse' can take more than four arguments.  If given more
 than four arguments, `ifelse' works like a `case' or `switch' statement
 in traditional programming languages.  If STRING-1 and STRING-2 are
 equal, `ifelse' expands into EQUAL-1, otherwise the procedure is
 repeated with the first three arguments discarded.  This calls for an
 example:
 
      ifelse(`foo', `bar', `third', `gnu', `gnats')
      error-->m4:stdin:1: Warning: excess arguments to builtin `ifelse' ignored
      =>gnu
      ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth')
      =>
      ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth', `seventh')
      =>seventh
      ifelse(`foo', `bar', `3', `gnu', `gnats', `6', `7', `8')
      error-->m4:stdin:4: Warning: excess arguments to builtin `ifelse' ignored
      =>7
 
    Naturally, the normal case will be slightly more advanced than these
 examples.  A common use of `ifelse' is in macros implementing loops of
 various kinds.
 

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