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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _compute_value(self, pkt):
 # type: (packet.Packet) -> int """ 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 """

        fld, fval = pkt.getfield_and_val(self._length_of)
        val = fld.i2len(pkt, fval)
        ret = self._adjust(val)
        assert(ret >= 0)
        return ret