Section One_predicate.
Variable P:A->Prop.
Inductive Exists : list A -> Prop :=
| Exists_cons_hd : forall x l, P x -> Exists (x::l)
| Exists_cons_tl : forall x l, Exists l -> Exists (x::l).
Hint Constructors Exists : core.

Lemma Forall_rect : forall (Q : list A -> Type),
Q [] -> (forall b l, P b -> Q (b :: l)) -> forall l, Forall l -> Q l.