proposition: Lemma injmK A : A \subset D -> f @*^-1 (f @* A) = A.
proof: Proof. by move=> sAD; rewrite morphimK // ker_injm // mul1g. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup)