Definition BDDvar_make (cfg : BDDconfig) (x : BDDvar) :=
  BDDmake cfg x BDDzero BDDone.

Lemma BDDvar_make_preserves_nodes : forall (cfg : BDDconfig) (x : BDDvar), BDDconfig_OK cfg -> nodes_preserved cfg (fst (BDDvar_make cfg x)).