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 (see section 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.
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