The ends matcher

You will write three small functions that decide whether two cut ends can be joined, and that count the copies of a cloned gene.

The idea. A site is written with a caret where the top strand is cut. The overhang is the part of the site between the cut and the middle of the site's other cut. Two sticky ends fit when both have an overhang and the overhangs are the same.

Your tasks.

1. Finish overhang. It gives the single-stranded end that is left, or an empty text for a blunt end.
2. Finish sticky_fit. It is True when both enzymes leave a sticky end and the two overhangs are equal.
3. Finish copies. It gives the copies of the gene in a culture, from the copies in one cell, the cells in one millilitre and the volume in millilitres.