Definition node_OK (bs : BDDstate) (node : ad) :=
    node = BDDzero \/ node = BDDone \/ in_dom _ node bs = true.

Lemma BDDalloc_allocates : forall (cfg : BDDconfig) (l r : ad) (x : BDDvar), MapGet _ (fst (fst (BDDalloc cfg x l r))) (snd (BDDalloc cfg x l r)) = Some (x, (l, r)).