proposition: Lemma pid_mx_row n r : pid_mx r = row_mx 1%:M 0 :> 'M_(r, r + n).
proof: Proof. apply/matrixP=> i j; rewrite !mxE ltn_ord andbT. by case: split_ordP => j' ->; rewrite !mxE// (val_eqE (lshift n i)) eq_shift. 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)