intros t; elim t; simpl in |- *; auto.
intros a; exists a; auto.
intros b (a, H); exists a; auto.
intros b (a, H); exists a; auto.
intros b (a, H) b0 H0; exists a; auto.
Qed.
(* Predicate for Partial Trees with Different Leaves *)
Definition distinct_pbleaves (t : pbtree) : Prop :=
forall t0 t1 t2 : pbtree,
inpb (pbnode t1 t2) t -> inpb t0 t1 -> inpb t0 t2 -> False.

(* A leaf tree has distinct leaves *)
Theorem distinct_pbleaves_Leaf : forall a : A, distinct_pbleaves (pbleaf a).
Proof using.