An elevator system is to be installed in a building with floors.

   1. Each elevator has buttons. Each elevator has one button for each floor. When a user presses a button, the button becomes illuminated and the elevator visits the corresponding floor. When the elevator visits a floor, the elevator cancels the corresponding illumination.
   2. up-elevator is an elevator.
   3. down-elevator is an elevator.
   4. Each floor has two buttons. (except ground and top). If the user presses the up-button, an up-elevator is requested. If the user presses the down-button, a down-elevator is requested. If the user presses a button, this button becomes illuminated. When the elevator visits a floor, the elevator cancels the corresponding illumination of the button in the desired direction. The system minimizes the waiting time.
   5. When an elevator has not to service any requests, the elevator remains at its final destination and the doors of the elevator are closed. The elevator awaits further requests.
   6. The elevators service all requests from floors eventually with equal priority.
   7. If a user presses a button within the elevator, the elevator services this request eventually in the direction of travel.

