proposition: Lemma conform_mx_id m n (B A : 'M_(m, n)) : conform_mx B A = A.
proof: Proof. by rewrite /conform_mx; do 2!case: eqP => // *; 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 countalg finalg zmodp)