intros a; red in |- *.
intros a0 t1 t2 H; inversion H.
Qed.
Hint Resolve distinct_leaves_leaf : core.

(* Specifications: *)
(* An inversion theorem for node *)
Theorem distinct_leaves_l :
forall t1 t2 : btree, distinct_leaves (node t1 t2) -> distinct_leaves t1.