public Parser scrollPhrase() {
        if (scrollPhraseParser == null) {
           FutureParser future = scoped("scrollPhrase");
           scrollPhraseParser = future;
           future.setParser(
               sequence(
                   token("SCROLL"),
                   choice(
                       token("UP"),
                       token("DOWN")
                   ),
                   optional(
                       sequence(
                           optional(
                               token("BY")
                           ),
                           choice(
                               integer(),
                               identifier()
                           ),
                           choice(
                               token("LINE"),
                               token("LINES")
                           )
                       )
                   )
               )
           );
        }

        return scrollPhraseParser;
    }