def __add__(self, other: Optional['SumMetric']) -> 'SumMetric':
    if (other is None):
        return self
    full_sum = (self._sum + other._sum)
    return type(self)(sum_=full_sum)
def __add__(self, other: Optional['SumMetric']) -> 'SumMetric':
    if (other is None):
        return self
    full_sum = (self._sum + other._sum)
    return type(self)(sum_=full_sum)