(a) Where a sole arbitrator is to be appointed and the parties have not agreed on an appointment procedure, the sole arbitrator shall be appointed jointly by the parties.
(b) If the appointment of the sole arbitrator is not made within the period of time agreed upon by the parties or, in the absence of such an agreed period of time, within 30 days after the commencement of the arbitration, the sole arbitrator shall be appointed in accordance with Article 19.