The plea based on the existence of an arbitration clause for contractual arbitration (arbitrato irrituale) constitutes a plea in the strict sense, having as its object the assertion of a fact impeding the exercise of state jurisdiction, with the consequence that it must be raised by the parties within the time and in the manner prescribed for pleas on the merits that cannot be raised of the court's own motion, and cannot be raised for the first time on appeal under art. 345 cod. proc. civ.