Definition bool_fun_impl (bf1 bf2 : bool_fun) : bool_fun :=
fun vb : var_binding => implb (bf1 vb) (bf2 vb).

Lemma BDDimpl_keeps_node_OK :
forall (cfg : BDDconfig) (negm : BDDneg_memo) (orm : BDDor_memo)
(node1 node2 : ad),
BDDconfig_OK cfg ->
BDDneg_memo_OK_2 cfg negm ->
BDDor_memo_OK cfg orm ->
config_node_OK cfg node1 ->
config_node_OK cfg node2 ->
forall node : ad,
config_node_OK cfg node ->
config_node_OK (fst (BDDimpl cfg negm orm node1 node2)) node.