public Parser relativeSubscript() {
        if (relativeSubscriptParser == null) {
           FutureParser future = scoped("relativeSubscript");
           relativeSubscriptParser = future;
           future.setParser(
               sequence(
                   identifier(),
                   choice(
                       token("+"),
                       token("-")
                   ),
                   integer()
               )
           );
        }

        return relativeSubscriptParser;
    }