fact: Definition comb_rel s t := ∃ ∃ $2 ≡ opair $0 $1 ∧ $3 ≡ opair (prep_string s $0) (prep_string t $1).
imports: Undecidability.FOL.ZF Undecidability.FOL.Syntax.Facts Undecidability.FOL.Semantics.Tarski.FullFacts Undecidability.FOL.Sets.ZF Lia Undecidability.PCP(PCP Util.PCP_facts Reductions.PCPb_iff_dPCPb) Undecidability(Shared.ListAutomation)