proposition: Lemma mxEmxcol m n (A : 'M[T]_(m, n)) : A = castmx (big_ord1 _ (fun=> m), erefl) (\mxcol__ A).
proof: Proof. by apply: trmx_inj; rewrite trmx_cast tr_mxcol [LHS]mxEmxrow. 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)