  Many results that are difficult can be found more easily by using a
generalization in the complex plane of Einstein's addition law of parallel
velocities. Such a generalization is a natural way to add quantities that are
limited to bounded values. We show how this generalization directly provides
phase factors such as the Wigner angle in special relativity and how this
generalization is connected in the simplest case with the composition of 2x2 S
matrices.
