public Parser external() {
        if (externalParser == null) {
           FutureParser future = scoped("external");
           externalParser = future;
           future.setParser(
               sequence(
                   optional(
                       token("IS")
                   ),
                   token("EXTERNAL"),
                   optional(
                       sequence(
                           choice(
                               token("AS"),
                               token("BY")
                           ),
                           literal()
                       )
                   )
               )
           );
        }

        return externalParser;
    }