Varset(const Varset& vs) : constant_(vs.constant_), cd_set_(vs.cd_set_), ncd_set_(vs.ncd_set_) {
Varset(Object constant, const Chain<StepVariable>* cd_set, const Chain<StepVariable>* ncd_set, Type type) : constant_(constant), cd_set_(cd_set), ncd_set_(ncd_set), type_(type) {
  Varset(const Varset& vs)    : constant_(vs.constant_), cd_set_(vs.cd_set_), ncd_set_(vs.ncd_set_) {    Chain<StepVariable>::register_use(cd_set_);    Chain<StepVariable>::register_use(ncd_set_);  }