code: def _pfp__build(self, stream=None, save_offset=False):
          
          if save_offset and stream is not None:
              self._pfp__offset = stream.tell()
  
          # returns either num bytes written or total data
          res = utils.binary("") if stream is None else 0
  
          # iterate IN ORDER
          for child in self._pfp__children:
              child_res = child._pfp__build(stream, save_offset)
              res += child_res
  
          return res
docs: Build the field and write the result into the stream
  
          :stream: An IO stream that can be written to
          :returns: None