proposition: Lemma incl_consE {X: Type} {a: X} {A: list X} {P: list X} : incl (a :: A) P -> In a P /\ incl A P.
proof: Proof. move=> H. constructor; move=> *; apply: H; by [left | right]. Qed.
imports: List Undecidability.PCP.PCP Undecidability.Synthetic.Definitions ssreflect ssrbool ssrfun