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

(* Direct subtrees of a tree with distinct leaves have distinct leaves *)
Theorem distinct_pbleaves_l :
forall t1 t2 : pbtree,
distinct_pbleaves (pbnode t1 t2) -> distinct_pbleaves t1.