Traffic Light

Build a traffic light that cycles through its colors when you press a button. You practice state for a position in a cycle.

• The complete interface: the current color and the Next button.
• Your job is the logic: know which color is next and move on to it.
• Keep the texts Red, Green, Yellow and Next, and the color's role="status": the tests use them.

• At the start the traffic light is on Red.
• Every time you press Next it moves to the next color of the list: Red, Green, Yellow.

• You open the challenge → you see Red.
• You press Next → you see Green.
• You press again → you see Yellow.

• State as the index of a cycle.
• Arithmetic to go around a cycle.