Definition bool_fun_if (x : BDDvar) (bf bf' : bool_fun) : bool_fun :=
  fun vb : var_binding => ifb (vb x) (bf vb) (bf' vb).

Lemma nodes_preserved_internal : forall (cfg cfg' : BDDconfig) (node : ad), BDDconfig_OK cfg -> BDDconfig_OK cfg' -> config_node_OK cfg node -> is_internal_node cfg' node -> is_internal_node cfg node.