code: def add_filter(self, new_filter, as_last=True):
          
          if as_last:
              self.filters.append(new_filter)
          else:
              self.filters = [new_filter] + self.filters
docs: Compose this filter with the given ``new_filter`` filter.
  
          :param new_filter: the filter to be composed
          :type  new_filter: :class:`~aeneas.textfile.TextFilter`
          :param bool as_last: if ``True``, compose to the right, otherwise to the left