  We consider perturbative quantum field theory in the causal framework. Gauge
invariance is, in this framework, an identity involving chronological products
of the interaction Lagrangian; it express the fact that the scattering matrix
must leave invariant the sub-space of physical states. We are interested in
generalizations of such identity involving Wick sub-monomials of the
interaction Lagrangian. The analysis can be performed by direct computation in
the lower orders of perturbation theory; guided by these computations we
conjecture a generalization for arbitrary orders.
