proposition: Lemma binunion_assoc x y z : (x ∪' y) ∪' z ≡' x ∪' (y ∪' z).
proof: Proof. Proof using M_ZF. apply M_ext; intros a [H|H] % binunion_el; eauto. - apply binunion_el in H as [H|H]; eauto. - apply binunion_el in H as [H|H]; eauto. Qed.
imports: Undecidability.FOL.ZF Undecidability.FOL.Syntax.Facts Undecidability.FOL.Semantics.Tarski.FullFacts Undecidability.FOL.Sets(ZF) Undecidability.FOL.Sets.ZF Undecidability.FOL.Reductions.PCPb_to_ZF Lia Undecidability.PCP(PCP Util.PCP_facts Reductions.PCPb_iff_dPCPb) Undecidability(Shared.ListAutomation) Morphisms