proposition: Lemma hsubmxK A : row_mx (lsubmx A) (rsubmx A) = A.
proof: Proof. by apply/matrixP=> i j /[!mxE]; case: split_ordP => k -> /[!mxE]. 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)