code: def _validate(self, writing=False):
          
          if (((len(self.fragment_offset) != len(self.fragment_length)) or
               (len(self.fragment_length) != len(self.data_reference)))):
              msg = ("The lengths of the fragment offsets ({len_offsets}), "
                     "fragment lengths ({len_fragments}), and "
                     "data reference items ({len_drefs}) must be the same.")
              msg = msg.format(len_offsets=len(self.fragment_offset),
                               len_fragments=len(self.fragment_length),
                               len_drefs=len(self.data_reference))
              self._dispatch_validation_error(msg, writing=writing)
          if any([x <= 0 for x in self.fragment_offset]):
              msg = "Fragment offsets must all be positive."
              self._dispatch_validation_error(msg, writing=writing)
          if any([x <= 0 for x in self.fragment_length]):
              msg = "Fragment lengths must all be positive."
              self._dispatch_validation_error(msg, writing=writing)
docs: Validate internal correctness.