code: def schedule(self, elements=None):
          
          if not elements:
              elements = self.all_my_hosts_and_services()
  
          # ask for service and hosts their next check
          for elt in elements:
              logger.debug("Add check for: %s", elt)
              self.add(elt.schedule(self.hosts, self.services, self.timeperiods,
                                    self.macromodulations, self.checkmodulations, self.checks))
docs: Iterate over all hosts and services and call schedule method
          (schedule next check)
  
          If elements is None all our hosts and services are scheduled for a check.
  
          :param elements: None or list of host / services to schedule
          :type elements: None | list
          :return: None