'Delete cells that are not storing a value.'

def _garbage_collect(self):

    delete_list = []
    for (coordinate, cell) in iteritems(self._cells):
        if ((cell.value in ('', None)) and (cell.comment is None) and (cell.style_id == 0)):
            delete_list.append(coordinate)
    for coordinate in delete_list:
        del self._cells[coordinate]
