proposition: Lemma nth_add_ribbon_lt_start i : i < start -> nth 0 res i = nth 0 sh i.
proof: Proof. move=> Hi; rewrite /add_ribbon_on nth_cat size_take. by rewrite -/(minn _ _) (minn_idPl lessz) Hi nth_take. Qed.
imports: HB(structures) mathcomp(all_ssreflect) tools sorted partition