proposition: Lemma cast_perm_morphM m n eq_m_n : {morph @cast_perm m n eq_m_n : x y / x * y >-> x * y}.
proof: Proof. by subst m. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)