Returns the current count of all documents, including the changes from
        the current changeMap.

def _current_size(self):
        """
        Returns the current count of all documents, including the changes from
        the current changeMap.
        """
        deletes, adds, _ = Watch._extract_changes(self.doc_map, self.change_map, None)
        return len(self.doc_map) + len(adds) - len(deletes)