proposition: Lemma delta_mx_lshift m n1 n2 i j : delta_mx i (lshift n2 j) = row_mx (delta_mx i j) 0 :> 'M_(m, n1 + n2).
proof: Proof. apply/matrixP=> i' j'; rewrite !mxE -(can_eq splitK) (unsplitK (inl _ _)). by case: split => ?; rewrite mxE ?andbF. 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)