  We re express the fermion's probability amplitude as a trace over spinor
indices, which formulation surprisingly does not exist in literature. This
formulation puts the probabilty amplitude and the the probabilty(squared
amplitude) of a given process on equal footing at the compuational level and
this is our principal motivation to write the present paper. We test the power
of the trace formula in three applications: Calculation of the charge-current
of fermions by using symbolic programs, which current so far was only
computable by hand, analytic compuation of the quark dipole magnetic moment,
rendered less cumbersome, and finally Fiertz rearrangement identities now made
more transparent.
