bool(trace) should handle default values

[sdementen]

    def __bool__(self):
        return bool(self._d)

should probably be rewritten as

    def __bool__(self):
        return bool(self._d) or bool(trace.default)

to see if is_empty should follow a similar logic with

    def is_empty(self):
        return len(self) == 0

rewritten as

    def is_empty(self):
        return len(self) == 0 and self.default is None

but I see it is used in bin() and check_boundaries() so not sure