Consider a street junction with 4 incoming roads. Each of the 4 roads has a traffic light, which are L1, L2, L3, L4.
The traffic light has 4 states: green, yellow, red, and amber which happen in that order.

Write a model of this junction with 4 traffic lights, showing a reasonable controller C that switches the green traffic lights in the following order: L1 and L3 together, then L2, then L4.

Use the variable names L1, L2, L3, L4, green, yellow, red, amber
