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 inductively_barred_at_imp_is_path_from :
forall P, demorgan_inductively_barred_at P -> forall n l,
~ inductively_barred_at P n l -> is_path_from P n l.