proposition: Lemma row_mxKl A1 A2 : lsubmx (row_mx A1 A2) = A1.
proof: Proof. by apply/matrixP=> i j; rewrite mxE row_mxEl. 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)