public Parser keyModifier() {
        if (keyModifierParser == null) {
           FutureParser future = scoped("keyModifier");
           keyModifierParser = future;
           future.setParser(
               sequence(
                   token("KEY"),
                   optional(
                       token("IS")
                   ),
                   choice(
                       generalRelationOp(),
                       token("FIRST"),
                       token("LAST")
                   ),
                   identifier(),
                   star(
                       sequence(
                           token("IN"),
                           identifier()
                       )
                   )
               )
           );
        }

        return keyModifierParser;
    }