code: def PrependUOffsetTRelative(self, off):
          
  
          # Ensure alignment is already done:
          self.Prep(N.UOffsetTFlags.bytewidth, 0)
          if not (off <= self.Offset()):
              msg = "flatbuffers: Offset arithmetic error."
              raise OffsetArithmeticError(msg)
          off2 = self.Offset() - off + N.UOffsetTFlags.bytewidth
          self.PlaceUOffsetT(off2)
docs: Prepends an unsigned offset into vector data, relative to where it
          will be written.