code: def unpack(self, buff, offset=0):
          
          super().unpack(buff, offset)
          # Recover field from field_and_hasmask.
          try:
              self.oxm_field = self._unpack_oxm_field()
          except ValueError as exception:
              raise UnpackException(exception)
  
          # The last bit of field_and_mask is oxm_hasmask
          self.oxm_hasmask = (self.oxm_field_and_mask & 1) == 1  # as boolean
  
          # Unpack oxm_value that has oxm_length bytes
          start = offset + 4  # 4 bytes: class, field_and_mask and length
          end = start + self.oxm_length
          self.oxm_value = buff[start:end]
docs: Unpack the buffer into a OxmTLV.
  
          Args:
              buff (bytes): The binary data to be unpacked.
              offset (int): If we need to shift the beginning of the data.