code: def _next_unit_in_service(service, placed_in_services):
      
      current = placed_in_services.get(service)
      number = 0 if current is None else current + 1
      placed_in_services[service] = number
      return number
docs: Return the unit number where to place a unit placed on a service.
  
      Receive the service name and a dict mapping service names to the current
      number of placed units in that service.