def _add_to(self, uuid, dest, other):
        """Add `item` to `dest` set, ensuring `item` is not present in `other`
        set."""
        _assert_uuid(uuid)
        try:
            other.remove(uuid)
        except KeyError:
            pass
        dest.add(uuid)
Add `item` to `dest` set, ensuring `item` is not present in `other`
        set.