code: function parseExtractor (inp) {
      var match = inp.takeAPeek(EXTRACTOR);
      if (match)
        return wrapped("(", ")", parsePattern, extractorRes, inp)
            || nodeExtractor(match[1]);
    
      function extractorRes (res) {
        return nodeExtractor(match[1], res);
      }
    }
docs: Parses custom extractors. Extractors are identifiers that begin with a dollar sign.