“…This is the content of a consistent literature on the subject [68,69,70,71,72,73,74,75,76] among which we would like to single out the more general, and recent, [77]: the problem is not trivial at all, because of the same subtleties that appear, for instance, in the Hamiltonian formulation of General Relativity. Among the various possible approaches we will closely follow the quantization procedure originally proposed by Farhi, Guth and Guven [16], which has, lately, been followed also in [67] and [78]. For the Lagrangian approach to the classical dynamics we will instead follow the clear and more general exposition of [77].…”