#[global]
Instance remove_rb s x : Rbt s -> Rbt (remove x s).(** ** Treeify *)

Definition treeify_rb_invariant size depth (f:treeify_t) :=
 forall acc,
 size <= length acc ->
  rbt depth (fst (f acc)) /\
  size + length (snd (f acc)) = length acc.

Lemma treeify_cont_rb f g size1 size2 size d : treeify_rb_invariant size1 d f -> treeify_rb_invariant size2 d g -> size = S (size1 + size2) -> treeify_rb_invariant size (S d) (treeify_cont f g).