Proof using.
intros l t1 t2 H H0; apply build_comp with (l1 := l) (l2 := l); auto.
exists l; exists l; simpl in |- *; auto.
Qed.

(* Building is compatible with permutation *)
Theorem build_permutation :
forall (l1 l2 : list (btree A)) (t : btree A),
build l1 t -> permutation l1 l2 -> build l2 t.