r separators like tabs, line ends, and form feeds may also
be declared irrelevant, and some applications may prefer to ignore the
distinction between upper and lower case input.</P>

<P>Comments are difficult to specify with the regular expressions used to
denote tokens - indeed, nested comments may not be specified at all in this
way. Since comments are usually discarded by a parsing process, and may
typically appear in arbitrary places in source code, it makes sense to have a
special construct to express 