StepDomain(size_t id, const VariableList& parameters, const ActionDomain& domain) : id_(id), parameters_(&parameters), domain_(&domain) {
StepDomain(const StepDomain& sd) : id_(sd.id_), parameters_(sd.parameters_), domain_(sd.domain_) { ActionDomain::register_use(domain_);
  StepDomain(size_t id, const VariableList& parameters,	     const ActionDomain& domain)    : id_(id), parameters_(&parameters), domain_(&domain) {  }