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 internal_node_lemma :
forall (cfg : BDDconfig) (node : ad),
BDDconfig_OK cfg ->
is_internal_node cfg node ->
Neqb (low cfg node) (high cfg node) = false /\
BDDbounded (fst cfg) (low cfg node) (var cfg node) /\
BDDbounded (fst cfg) (high cfg node) (var cfg node).