proposition: Lemma col_mxEu A1 A2 i j : col_mx A1 A2 (lshift m2 i) j = A1 i j.
proof: Proof. by rewrite mxE (unsplitK (inl _ _)). 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)