Keywords can be used only as indicated by the context-free grammar
    productions.<p>

    Comments are arbitrary sequences of characters beginning with
    ``(*'' and ending with ``*)''.  Comments cannot be nested.
    Comments, spaces, and newlines may not appear within basic
    symbols.  Two adjacent basic symbols must be separated by one or
    more comments, spaces, or newlines, unless one of the basic
    symbols is a special character.  Otherwise comments, spaces, and
    newlines are meaningless.
