FIREAddPropagators is an option for
FIREBurn. Normally, for loop integrals that do not have
enough propagators to form a complete basis, FIREBurn will
automatically include missing propagators and put them to unity after
the reduction is complete. In some cases it may be desirable to choose
the missing propagators manually. This can be done by specifying the
propagators via AddPropagators->{prop1,prop2,...}.