Validity of an arbitration clause inserted in a contract for the purchase and sale of electric power between a mixed-economy company (Companhia Estadual de Energia Elétrica - CEEE) and a private company. Disposable patrimonial right. "Where the object of the public service contract performed by the state entity is strictly linked to the economic activity carried out by the state-owned company — in this case, the sale of electric power — there is no impediment to the stipulation of the respective arbitration clause":