Ltac omega_bal := match goal with
  | H:avl ?l, H':avl ?r |- context [ bal ?l ?x ?e ?r ] =>
     generalize (bal_height_1 x e H H') (bal_height_2 x e H H');
     omega_max
  end.

Lemma remove_min_avl : forall l x e r h, avl (Node l x e r h) -> avl (remove_min l x e r)#1.