proposition: Lemma morphim_restrm B : fA @* B = f @* (A :&: B).
proof: Proof. by rewrite {2}/morphim setIA (setIidPr sAD). Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup)