ExceptHeads is an option of FCLoopIsolate,
FCDiracIsolate and other functions. It takes a list of
heads that are not allowed to appear inside isolated expression.
For example, ExceptHeads -> {DiracGamma} in
FCLoopIsolate blocks loop integrals where loop momenta are
contracted with Dirac matrices.
Overview, FCLoopIsolate, FCDiracIsolate.