Mandelstam is an option for DiracTrace,
OneLoop, OneLoopSum and
TrickMandelstam. A typical setting is
Mandelstam -> {s, t, u, m1^2+m2^2+m3^2+m4^2}, which
implies s + t + u =
m_1^2+m_2^2+m_3^2+m_4^2. If other than four-particle processes
are calculated, the setting should be
Mandelstam -> {}.
Overview, DiracTrace, OneLoop, OneLoopSum, TrickMandelstam, SetMandelstam.