by move=> x /setDP[_ /cfun0->]; rewrite mul0r.
  by congr (_ * _); apply: eq_bigr => x Hx; rewr

(* Specifications: *)
transitivity (h' * \sum_(y in G) \sum_(x in G) phi (x ^ y) * (psi (x ^
have [Hxz | notHxz] := boolP (x ^ z \in H); first by rewrite cfMorphE.
Proof.