Trophic levels from a web

You will write the function that finds the trophic levels a species can occupy, and a function that finds the longest chain.

The idea. The producer is at level 1. An animal is one level above each of its foods, so its set of levels is the levels of its foods, each plus one.

Your tasks.

1. Finish levels. It returns the set of the trophic levels of a species.
2. Finish longest. It returns the highest level that any species of the web can reach.