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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 pack(self):
 """ Pack message to binary stream. """

        payload = io.BytesIO()
        # Advance num bytes equal to header size - the header is written later
        # after the payload of all segments and parts has been written:
        payload.seek(self.header_size, io.SEEK_CUR)

        # Write out payload of segments and parts:
        self.build_payload(payload)

        packet_length = len(payload.getvalue()) - self.header_size
        self.header = MessageHeader(self.session_id, self.packet_count, packet_length, constants.MAX_SEGMENT_SIZE,
                                    num_segments=len(self.segments), packet_options=0)
        packed_header = self.header_struct.pack(*self.header)

        # Go back to begining of payload for writing message header:
        payload.seek(0)
        payload.write(packed_header)
        payload.seek(0, io.SEEK_END)

        trace(self)

        return payload