code: def _compute_value(self, pkt):
          # type: (packet.Packet) -> int
          
          fld, fval = pkt.getfield_and_val(self._length_of)
          val = fld.i2len(pkt, fval)
          ret = self._adjust(val)
          assert(ret >= 0)
          return ret
docs: Computes the value of this field based on the provided packet and
          the length_of field and the adjust callback
  
          @param packet.Packet pkt: the packet from which is computed this field value.  # noqa: E501
          @return int: the computed value for this field.
          @raise KeyError: the packet nor its payload do not contain an attribute
            with the length_of name.
          @raise AssertionError
          @raise KeyError if _length_of is not one of pkt fields