>>> from divide import add_avg

>>> l = [3, 7]
>>> add_avg(l)
>>> l == [3, 7, 5.0]
True
>>> l = [-2]
>>> add_avg(l)
>>> l == [-2, -2.0]
True
>>> l = [3, 7, 2]
>>> add_avg(l)
>>> l == [3, 7, 2, 4.0]
True
>>> l = [-2, 0]
>>> add_avg(l)
>>> l == [-2, 0, -1.0]
True
>>> l = []
>>> add_avg(l)
>>> l == [0.0]
True