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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 icmpv6(self):
 """ - An ICMPv6Header instance, if the packet is valid ICMPv6. - None, otherwise. """

        ipproto, proto_start = self.protocol
        if ipproto == Protocol.ICMPV6:
            return ICMPv6Header(self, proto_start)