proposition: Lemma row_mxEl A1 A2 i j : row_mx A1 A2 i (lshift n2 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)