proposition: Lemma delta_mx_rshift m n1 n2 i j : delta_mx i (rshift n1 j) = row_mx 0 (delta_mx i j) :> 'M_(m, n1 + n2).
proof: Proof. apply/matrixP=> i' j'; rewrite !mxE -(can_eq splitK) (unsplitK (inr _ _)). 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)