proposition: Lemma castmx_id m n erefl_mn (A : 'M_(m, n)) : castmx erefl_mn A = A.
proof: Proof. by case: erefl_mn => e_m e_n; rewrite [e_m]eq_axiomK [e_n]eq_axiomK. 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)