#[global]
Instance join_ok : forall l x r `(Ok l, Ok r, lt_tree x l, gt_tree x r),
 Ok (join l x r).

Lemma remove_min_spec : forall l x r y h, InT y (Node h l x r) <-> X.eq y (remove_min l x r)#2 \/ InT y (remove_min l x r)#1.