proposition: Lemma eqmx_cast m1 m2 n (A : 'M_(m1, n)) e : ((castmx e A : 'M_(m2, n)) :=: A)%MS.
proof: Proof. by case: e A; case: m2 / => A e; rewrite castmx_id. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg finalg zmodp matrix)