def parse(scanner, rules, scope = ParserScope.empty)
      scanner.scan Regexp.compile(Regexp.escape scope[ref_label])
    end
def parse(scanner, rules, scope = ParserScope.empty)
      scanner.scan Regexp.compile(Regexp.escape scope[ref_label])
    end
If the earlier referenced match result appears again at the parse
 position, match that string, otherwise return a false value.

 @param (see Match#parse)

 @return the matched string, or +nil+