public Parser unitPhrase() {
        if (unitPhraseParser == null) {
           FutureParser future = scoped("unitPhrase");
           unitPhraseParser = future;
           future.setParser(
               sequence(
                   token("UNIT"),
                   choice(
                       identifier(),
                       literal()
                   )
               )
           );
        }

        return unitPhraseParser;
    }