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.