Definition nodes_preserved (cfg cfg' : BDDconfig) :=
  forall (x : BDDvar) (l r node : ad),
  MapGet _ (fst cfg) node = Some (x, (l, r)) ->
  MapGet _ (fst cfg') node = Some (x, (l, r)).

Lemma BDDmake_preserves_nodes : forall (cfg : BDDconfig) (x : BDDvar) (l r : ad), BDDconfig_OK cfg -> nodes_preserved cfg (fst (BDDmake cfg x l r)).