proposition: Lemma morphim_invm A : A \subset G -> invm @* (f @* A) = A.
proof: Proof. by move=> sAG; rewrite morphim_factm morphim_idm. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup)