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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 post_build(self, pkt, pay):
 """ Implements the swap-bytes functionality when building this is based on a copy of the Packet.self_build default method. The goal is to affect only the CAN layer data and keep under layers (e.g LinuxCooked) unchanged """

        if conf.contribs['CAN']['swap-bytes']:
            return CAN.inv_endianness(pkt) + pay
        return pkt + pay