Arbitration clause designating an arbitration center (CMA-PR). Failure of one of the parties to appear at the signing of the agreement to arbitrate. Necessity of proving that the rules of the institution authorized the commencement of the arbitration proceedings even in the absence of one of the parties for the execution of the agreement to arbitrate. The absence of such proof requires the filing of the lawsuit provided for in Article 7. The appearance of the party at a hearing in the arbitration proceedings does not cure such absence: