“…However, it is known that the most relativistic interactions are those where the spin is taken into account which is a very useful and very important notion in physics. From a practical point of view, the explicit calculus of propagators for such interactions by the path-integral formalism, has been discussed by many authors previously ( [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]). …”