'Add counts from two counters.
>>> Counter(\'abbb\') + Counter(\'bcc\')
Counter({\'b\': 4, \'c\': 2, \'a\': 1})'

def __add__(self, other):

    if (not isinstance(other, Counter)):
        return NotImplemented
    result = Counter()
    for elem in (set(self) | set(other)):
        newcount = (self[elem] + other[elem])
        if (newcount > 0):
            result[elem] = newcount
    return result
