public Parser summand() {
        if (summandParser == null) {
           FutureParser future = scoped("summand");
           summandParser = future;
           future.setParser(
               sequence(
                   factor(),
                   star(
                       sequence(
                           choice(
                               token("*"),
                               token("/")
                           ),
                           factor()
                       )
                   )
               )
           );
        }

        return summandParser;
    }