(a) If the parties have agreed on a procedure for the appointment of the arbitrator or arbitrators other than as envisaged in Articles 16 to 20, that procedure shall be followed.
(b) If the Tribunal has not been established pursuant to such procedure within the period of time agreed upon by the parties or, in the absence of such an agreed period of time, within 45 days after the commencement of the arbitration, the Tribunal shall be established or completed, as the case may be, in accordance with Article 19.