SetStandardMatrixElements[{sm1 -> abb1}, {sm2 -> abb2}, ...]
sets abbreviations abb1, abb2, ... for matrix elements
sm1, sm2, ....
SetStandardMatrixElements[{sm1 -> abb1}, {sm2 -> abb2}, …,
cons]. Set abbreviations abb1, abb2, ... for matrix
elements sm1, sm2, ... using energy-momentum conservation
cons, e.g. k2 -> p1 + p2 - k1