Follow the blood

A list gives the stops of one round of the blood and the oxygen at each stop. You will move from stop to stop, name the loop of each stop and pick out the stops where the blood is rich in oxygen.

The idea. Stops 1 to 4 belong to the pulmonary loop. Stops 5 to 8 belong to the systemic loop. The round ends where it began.

Your tasks.

1. Finish next_stop. It gives the stop after a given stop, and the first stop after the last one.
2. Finish circuit. It gives "pulmonary" or "systemic" for a stop.
3. Set rich to the list of the stops where the oxygen is "high", in order.