The chapter matcher

A dictionary holds the pairs of the chapter. You will look up the host or source of a product, find a year, and count the wrong pairs in a student's list.

The idea. Each application has one organism or tool and one fact. A pair is wrong if it does not agree with the dictionary.

Your tasks.

1. Finish source_of. It gives the organism or tool for a product, or "unknown" if the product is not in the dictionary.
2. Finish products_of_year. It gives the list of products that go with a year, in the order of the dictionary.
3. Finish wrong_pairs. It gives the number of pairs in a list of (product, source) that do not agree with the dictionary.