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.

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()