proposition: Lemma submxrowK m (A : 'M[T]_(m, sq)) : \mxrow_j (submxrow A j) = A.
proof: Proof. by apply/matrixP => k l; rewrite !mxE !sig2K. 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)