code: def insert_chunk(self, id_):
          
  
          assert_valid_chunk_id(id_)
  
          self.__fileobj.seek(self.__next_offset)
          self.__fileobj.write(pack('>4si', id_.ljust(4).encode('ascii'), 0))
          self.__fileobj.seek(self.__next_offset)
          chunk = IFFChunk(self.__fileobj, self[u'FORM'])
          self[u'FORM']._update_size(self[u'FORM'].data_size + chunk.size)
  
          self.__chunks[id_] = chunk
          self.__next_offset = chunk.offset + chunk.size
docs: Insert a new chunk at the end of the IFF file