Extend the Dutch traffic light depicted below to behave like a German traffic light. The German traffic light has an extra phase between the red and green switching by first displaying a red/yellow combination before switching to green (Red => Red/Yellow => Green => Yellow).
a) Give a Petri net that is able of behaving like a German traffic light. There should be three places that indicate the current colour and all of the state transitions given above should be supported (cyclic).
b) Refine the Petri net to behave exactly like a German traffic light; e.g. no other state transitions than given above should be supported.
c) Create a group of two German traffic lights that behave safe and fair. Safe means that at least one traffic light must be red at any time. Fair means that the traffic lights alternate in turning green.