com - the cmusic comment command

The command

    com This is a comment ;

may be used to place comments into a cmusic score (this is for
backwards compatibility with MUSIC V).

The preferred way to place comments in a cmusic score is between
curly braces, which may be nested, such as

    {
	This is 
	    {really it is} 
	a comment in a cmusic score
    }

The advantage is nest-able comments is that it is possible to "comment
out" a portion of a score for a test run.  The "commented out" portion
might itself contain comments, so nesting is useful.
