(c) An international arbitrator or ad hoc arbitral tribunal to be appointed by a special agreement or established under the Arbitration Rules of the United Nations Commission on International Trade Law. 3. If after a period of three (3) months from written notification of the claim there is no agreement to one of the above alternative procedures, the parties to the dispute shall be bound to submit the dispute to arbitration under the Arbitration Rules of the United Nations Commission on International Trade Law as then in force. The parties to the dispute may agree in writing to modify these Rules. 4. The provisions of this Article shall not apply to a dispute, which arose before entry into force of this Annex. 36