Action to set aside an arbitration award. Controversy regarding the stabilization of the claim in arbitration. Expansion of the scope of the arbitration occurring after the signing of the terms of reference. Admissibility. Logical-systematic interpretation of the claims and their natural consequences. Rules of the arbitral institution allowing modification of the disputed subject matter after the terms of reference, provided that effective adversarial proceedings were ensured. Observance of the adversarial principle regarding the scope of the arbitration. Validity of the arbitration award: