A group is defined as one or more stones of the same colour, that are
connected along the lines. Stones besides each other are connected,
but stones positioned diagonally from each other are not.

A liberty is an empty space beside a stone. Again, an empty space diagonally
from a stone does not count.

The liberties of a group, are the number of liberties that a group have.

Liberties can be shared by more than one group.