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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 crosslisted_with(self, crn):
 """Returns all the CRN courses crosslisted with the given crn. The returned crosslisting does not include the original CRN. """

        raise NotImplemented
        return tuple([c for c in self.crosslistings[crn].crns if c != crn])