public Parser locationPhrase() {
        if (locationPhraseParser == null) {
           FutureParser future = scoped("locationPhrase");
           locationPhraseParser = future;
           future.setParser(
               sequence(
                   choice(
                       token("BEFORE"),
                       token("AFTER")
                   ),
                   optional(
                       token("INITIAL")
                   ),
                   choice(
                       identifier(),
                       literal()
                   )
               )
           );
        }

        return locationPhraseParser;
    }