From Undecidability.PCP.Reductions Require
SR_to_MPCP MPCP_to_MPCPb MPCP_to_PCP PCP_to_PCPb PCPb_iff_iPCPb PCPb_iff_dPCPb.
Require Undecidability.StringRewriting.SR_undec.
(** ** PCP is undecidable *)
Check MPCP_undec.
Check MPCPb_undec.

(* The Post correspondence problem is undecidable. *)
Lemma PCP_undec : undecidable PCP.