Dynamic rules

• Tokens that use double quoted strings or regexes do interpolation.
• Hence, they can be make context sensitive, by referring to other items in the production rule. For instance, by interpolating $item.
• The <matchrule:...> is another way of building dynamic rules. The part after the colon is evaluated as a double quoted string each time the directive is encountered, and the value it evaluates to is taken as the name of the subrule to be matched.