public Parser factor() {
        if (factorParser == null) {
           FutureParser future = scoped("factor");
           factorParser = future;
           future.setParser(
               sequence(
                   optional(
                       unaryOperator()
                   ),
                   atomicExpression(),
                   star(
                       sequence(
                           token("**"),
                           optional(
                               unaryOperator()
                           ),
                           atomicExpression()
                       )
                   )
               )
           );
        }

        return factorParser;
    }