'Inplace add from another counter, keeping only positive counts.
>>> c = Counter(\'abbb\')
>>> c += Counter(\'bcc\')
>>> c
Counter({\'b\': 4, \'c\': 2, \'a\': 1})'

def __iadd__(self, other):

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