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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):
 """Called to create a STOMP message from the internal values. """

        headers = ''.join(
            ['%s:%s\n' % (f, v) for f, v in sorted(self.headers.items())]
        )
        stomp_message = "%s\n%s\n%s%s\n" % (self._cmd, headers, self.body, NULL)

#        import pprint
#        print "stomp_message: ", pprint.pprint(stomp_message)

        return stomp_message