proposition: Lemma modactE x a : a \in D -> a \in 'N(H) -> x \in range -> modact x (coset H a) = to x a.
proof: Proof. by move=> Da Na Rx; rewrite modactEcond ?Rx // inE Da. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)