docs: def add(self, si):
          '''puts `si` into the currently open chunk, which it creates if
          necessary.  If this item causes the chunk to cross chunk_max,
          then the chunk closed after adding.
  
          '''
          if self.o_chunk is None:
              if os.path.exists(self.t_path):
                  os.remove(self.t_path)
              self.o_chunk = streamcorpus.Chunk(self.t_path, mode='wb')
          self.o_chunk.add(si)
          logger.debug('added %d-th item to chunk', len(self.o_chunk))
          if len(self.o_chunk) == self.chunk_max:
              self.close()