code: public final Parser<String> source() {
      return new Parser<String>() {
        @Override boolean apply(ParseContext ctxt) {
          int begin = ctxt.getIndex();
          if (!Parser.this.apply(ctxt)) {
            return false;
          }
          ctxt.result = ctxt.source.subSequence(begin, ctxt.getIndex()).toString();
          return true;
        }
        @Override public String toString() {
          return "source";
        }
      };
    }
docs: A {@link Parser} that returns the matched string in the original source.