proposition: Lemma mxEmxblock m n (A : 'M[T]_(m, n)) : A = castmx (big_ord1 _ (fun=> m), big_ord1 _ (fun=> n)) (\mxblock_(i < 1, j < 1) A).
proof: Proof. by rewrite [LHS]mxEmxrow mxrowEblock castmx_comp; apply: eq_castmx. 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)