DeclarePlusMinusOne

Set many symbols to be treated as +1 or -1:

Set all expressions matching given pattern to be treated as +1 or -1:

Keep polarization of polarization vector symbolic but restrict it to transverse case:

PolVec[P, pol] is not treated as massless spinor since pol can represent any polarization including 0:

Some automatic simplifications are not performed e.g. such spinor chain stays nonzero:

Declare pol symbol to be treated as plus or minus one:

After setting pol to represent plus or minus one Mathematica knows that PolVec[P, pol] represents massless spinor:

Package can use automatic simplifications:

Keep mass sign of massive spinor label symbolic:

SpM[P, x] is not treated as proper massive spinor label since x is not interpreted as +1 or -1:

Declare x symbol to be treated as plus or minus one:

After setting x to represent plus or minus one Mathematica knows that SpM[P, x] represents massive spinor label: