'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, count) in self.items():
        newcount = (count + other[elem])
        if (newcount > 0):
            result[elem] = newcount
    for (elem, count) in other.items():
        if ((elem not in self) and (count > 0)):
            result[elem] = count
    return result
