proposition: Lemma pid_mx_col m r : pid_mx r = col_mx 1%:M 0 :> 'M_(r + m, r).
proof: Proof. apply/matrixP=> i j; rewrite !mxE andbC. by case: split_ordP => ? ->; rewrite !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)