Validity of the arbitration clause granting the arbitrator the power to "define the procedure to be adopted" and authorizing judgment by equity, which implies the impossibility of requiring a more in-depth analysis of technical-accounting issues by the arbitration award. The challenge to an arbitrator by one of the parties once the arbitration proceedings have already been initiated may only occur if the grounds therefor became known after the appointment. Extent and depth of the relationship between the arbitrator and the party that appointed him, which were previously known. Failure to prove the arbitrator's lack of impartiality. Absence of non-compliance with the guiding principles of the arbitration proceedings: