public Parser ifStatement() {
        if (ifStatementParser == null) {
           FutureParser future = scoped("ifStatement");
           ifStatementParser = future;
           future.setParser(
               sequence(
                   token("IF"),
                   condition(),
                   optional(
                       token("THEN")
                   ),
                   choice(
                       nestedStatements(),
                       sequence(
                           token("NEXT"),
                           token("SENTENCE")
                       )
                   ),
                   optional(
                       sequence(
                           token("ELSE"),
                           choice(
                               nestedStatements(),
                               sequence(
                                   token("NEXT"),
                                   token("SENTENCE")
                               )
                           )
                       )
                   ),
                   optional(
                       token("END-IF")
                   )
               )
           );
        }

        return ifStatementParser;
    }