Definition bool_fun_independent (bf : bool_fun) (x : BDDvar) :=
  forall vb : var_binding,
  bool_fun_eval bf (augment vb x true) =
  bool_fun_eval bf (augment vb x false).

Lemma low_bounded : forall (cfg : BDDconfig) (node : ad), BDDconfig_OK cfg -> is_internal_node cfg node -> BDDbounded (fst cfg) (low cfg node) (var cfg node).