The route finder

A dictionary tells which part of the male system comes after each part. You will follow it to write the route of a sperm, count the steps between two parts, and find the parts that a block would cut off.

The idea. The route is a chain. Each part points to the next one, until the sperm leaves the body.

Your tasks.

1. Finish route_from. It gives the list of parts from start to the urethral opening, in order.
2. Finish steps_between. It gives how many steps lie between two parts on the route.
3. Finish cut_off. It gives the parts that come after a blocked part, in order.