proposition: Lemma enc_stack_el' x A : x ∈ M_enc_stack A -> exists s t, (s, t) el A /\ x ≡' M_enc_card s t.
proof: Proof. Proof using M_ZF. induction A as [|[s t] A IH]; cbn. - now intros H % M_eset. - intros [H|H] % binunion_el. + destruct (IH H) as (u&v&H1&H2). exists u, v. intuition. + apply sing_el in H. exists s, t. intuition. 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