Returns all the CRN courses crosslisted with the given crn.
        The returned crosslisting does not include the original CRN.

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])