Section Well_founded.
Variable A : Type.
Variable R : A -> A -> Prop.
(** The accessibility predicate is defined to be non-informative *)
(** (Acc_rect is automatically defined because Acc is a singleton type) *)
Inductive Acc (x: A) : Prop :=
Acc_intro : (forall y:A, R y x -> Acc y) -> Acc x.

Lemma Acc_inv : forall x:A, Acc x -> forall y:A, R y x -> Acc y.