proposition: Lemma colsub_cast m n1 n2 (eq_n : n1 = n2) (A : 'M_(m, n2)) : colsub (cast_ord eq_n) A = castmx (erefl, esym eq_n) A.
proof: Proof. by case: _ / eq_n in A *; apply: (mxsub_eq_id _ (cast_ord_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)