proposition: Lemma map_delta_mx i j : (delta_mx i j)^f = delta_mx i j :> 'M_(m, n).
proof: Proof. by apply/matrixP=> i' j'; rewrite !mxE rmorph_nat. 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)