The Weyl-van der Waerden spinor technique for calculating helicity amplitud
es of massive and massless particles is presented in a form that is particu
larly well suited to a direct implementation in computer algebra. Moreover,
we explain how to exploit discrete symmetries and how to avoid unphysical
poles in amplitudes in practice. The efficiency of the formalism is demonst
rated by giving explicit compact results for the helicity amplitudes of the
processes gamma gamma -->f (f) over bar, f (f) over bar --> gamma gamma ga
mma, mu(-) mu(+) --> f (f) over bar gamma. [S0556-2821(99)02101-3].