 The model just presented (QMSL) has a serious drawback: it does not allow to deal with systems containing identical constituents, because it does not respect the symmetry or antisymmetry requirements for such particles. A quite natural idea to overcome this difficulty is to relate the hitting process not to the individual particles but to the particle number density averaged over an appropriate volume. This can be done by introducing a new phenomenological parameter in the theory which however can be eliminated by an appropriate limiting procedure (see below).