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(m4.info)Dnl

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Deleting whitespace in input
============================

   The builtin `dnl' reads and discards all characters, up to and
including the first newline:

     dnl

and it is often used in connection with `define', to remove the newline
that follow the call to `define'.  Thus

     define(`foo', `Macro `foo'.')dnl A very simple macro, indeed.
     foo
     =>Macro foo.

   The input up to and including the next newline is discarded, as
opposed to the way comments are treated (Note: Comments).

   Usually, `dnl' is immediately followed by an end of line or some
other whitespace.  GNU `m4' will produce a warning diagnostic if `dnl'
is followed by an open parenthesis.  In this case, `dnl' will collect
and process all arguments, looking for a matching close parenthesis.
All predictable side effects resulting from this collection will take
place.  `dnl' will return no output.  The input following the matching
close parenthesis up to and including the next newline, on whatever
line containing it, will still be discarded.