To do approximation in amplitude square calculation

It is needed to do some approximation to simplify calculation in some complicate case, To do it, the user can add the following table

where the first line means to drop terms contains ${cnew}^2$, the last line means to drop electron's mass in amplitude square. It is easy to add more line to drop more terms. It will cause divergence to drop fermion's mass in the case if there is t-channel singularity related to the fermion. Therefor, a safe way to do it is introduced in the following section.