proposition: Fact vec_pos_app_right v w i : vec_pos (vec_app v w) (pos_right _ i) = vec_pos w i.
proof: Proof. unfold vec_app; rewrite vec_pos_set, pos_both_right; auto. Qed.
imports: Arith Lia List Permutation Undecidability.Shared.Libs.DLW.Utils(utils) Undecidability.Shared.Libs.DLW.Vec(pos)