Definition demorgan_or (P:list bool -> Prop) l l' := ~ (P l /\ P l') -> ~ P l \/ ~ P l'.Definition demorgan_inductively_barred_at P :=
  forall n l, demorgan_or (inductively_barred_at P n) (true::l) (false::l).

Lemma Y_unique : forall P, demorgan_inductively_barred_at P -> forall l1 l2, length l1 = length l2 -> Y P l1 -> Y P l2 -> l1 = l2.