proposition: Lemma castmxK m1 n1 m2 n2 (eq_m : m1 = m2) (eq_n : n1 = n2) : cancel (castmx (eq_m, eq_n)) (castmx (esym eq_m, esym eq_n)).
proof: Proof. by case: m2 / eq_m; case: n2 / eq_n. 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)