'See docstring of SeqBase._add'

def _add(self, other):

    if isinstance(other, SeqPer):
        (per1, lper1) = (self.periodical, self.period)
        (per2, lper2) = (other.periodical, other.period)
        per_length = lcm(lper1, lper2)
        new_per = []
        for x in range(per_length):
            ele1 = per1[(x % lper1)]
            ele2 = per2[(x % lper2)]
            new_per.append((ele1 + ele2))
        (start, stop) = self._intersect_interval(other)
        return SeqPer(new_per, (self.variables[0], start, stop))
