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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, packet, payload):
 """Compute the 'records_number' field when needed"""

        if self.records_number is None:
            recnum = struct.pack("!H", len(self.records))
            packet = packet[:6] + recnum + packet[8:]
        return _ICMPv6.post_build(self, packet, payload)