proposition: Lemma FST_pair_el x y z : FSTeq ⊢ (z ≡ x ∨ z ≡ y) -> FSTeq ⊢ z ∈ {x; y}.
proof: Proof. now apply FST_pair_el'. Qed.
imports: Undecidability.FOL.Syntax.Facts Undecidability.FOL.Semantics.Tarski.FullFacts Undecidability.FOL.Deduction.FullNDFacts Undecidability.FOL.Sets.FST Undecidability.FOL.FST Undecidability.FOL.Reductions.PCPb_to_FST Undecidability.PCP(PCP Util.PCP_facts Reductions.PCPb_iff_dPCPb) Undecidability(Shared.ListAutomation)