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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 'sources_number' field when needed"""

        if self.sources_number is None:
            srcnum = struct.pack("!H", len(self.sources))
            packet = packet[:26] + srcnum + packet[28:]
        return _ICMPv6.post_build(self, packet, payload)