no_change_to_t_photon

The option is used to controle the way to deal with cancellation due to gauge invarince in the case of t-channel photon connected to fermion with small mass[#_#>. The default choose is to do the projection to the fermion line. The user can choose not to do any projection to the fermion line by set it to:

       no_change_to_t_photon:='ok;

In the case of gauge invariance keeping, there will be the same reuslts from the different choose, but in the case of gauge invariance breaking by the width of unstable particles in the propagator formular, The result of no projection is wrong while the result with the projection is the right one.