'Inplace subtract counter, but keep only results with positive counts.
>>> c = Counter(\'abbbc\')
>>> c -= Counter(\'bccd\')
>>> c
Counter({\'b\': 2, \'a\': 1})'

def __isub__(self, other):

    for (elem, count) in other.items():
        self[elem] -= count
    return self._keep_positive()
