Proof using.
intros t; elim t; simpl in |- *; auto.
intros b H b0 H0 H1; red in |- *.
intros t0 t1 t2 H2; inversion H2.
intros H4 H7; case (inb_ex t0); intros a HH.
apply ulist_app_inv with (a := a) (1 := H1); auto; apply all_leaves_in;
apply inb_trans with (1 := HH); auto.
apply H; auto; try apply ulist_app_inv_l with (1 := H1).
apply H0; auto; try apply ulist_app_inv_r with (1 := H1).
Qed.

(* If the tree has distinct leaves then the list of all the leaves of the tree is unique *)
Theorem all_leaves_ulist :
forall t, distinct_leaves t -> ulist (all_leaves t).