an interesting problem to try and model:
a 20 storey building with two elevators, each can move independantly and at the same time in different directions, all features must be present (doors open close etc) and each elevator must be able to be called or sent to different floor at the same time.
i tried to solve this while bored one day and after several weeks i gave up , i dont think real time systems prgramming is for me.
have a try