proposition: Theorem bt_cup_spec x s t : x ∈ s ∪ t <-> x ∈ s \/ x ∈ t.
proof: Proof. revert x; induction s as [ | y s Hs ] using bt_rect'; intros x; simpl; btm simpl. rewrite Hs; tauto. Qed.
imports: List Arith Lia Max Wellfounded Setoid Eqdep_dec Undecidability.Shared.Libs.DLW.Utils(utils_tac utils_list utils_decidable finite seteq) Undecidability.Shared.Libs.DLW.Vec(pos) Undecidability.Shared.Libs.DLW.Wf(wf_finite wf_chains) Undecidability.FOL.TRAKHTENBROT(notations fol_ops)