That was my original idea, to be able to access the property-as-set and do just add/discard.
But the getter has to eval the set every time.

``` python
    def _set_hdocset(self, value):
        """  
        Setter for the hdocs-set for this mailbox.
        """
        hdocset_doc = self._get_hdocset_doc()
        newv = set(value)
        self.__hdocset = newv 

        with self._hdocset_lock:
            hdocset_doc.content[fields.HDOCS_SET_KEY] = list(newv)
            # XXX should deferLater 0 it?
            self._soledad.put_doc(hdocset_doc)
```