def symmetric_difference(self, other):
        """
        Return a tree with elements only in self or other but not
        both.
        """
        if not isinstance(other, set): other = set(other)
        me = set(self)
        ivs = me.difference(other).union(other.difference(me))
        return IntervalTree(ivs)
Return a tree with elements only in self or other but not
        both.