For the purposes of the admissibility of a challenge to an award, an inaccurate reference to the various heads of nullity provided for by art. 829 cod. proc. civ. is immaterial, provided that the breaches of law allegedly committed by the arbitrators are set out, in compliance with the requirements of that provision in the version applicable at the time the arbitration agreement was concluded. The foundation of any arbitration lies in the free choice of the parties, which alone is capable of derogating from the constitutional precept on the exercise of the judicial function, so that the source of arbitration cannot be found in an ordinary statute or, more generally, in an authoritative act of will. An arbitration clause contained in an agreement ancillary to a concession, drafted and imposed by the public party, does not constitute mandatory arbitration, since that party thereby freely and in advance expressed its intention to submit to arbitration, and the asymmetry of the right to decline arbitration remains rational and justified. Absent any foothold in the wording of the arbitration clause, the right to decline arbitration conferred on the concessionaire cannot be regarded as also conferred on the public administration.