def get_composition_repository_assignment_session(self, proxy):
        """Gets the session for assigning composition to repository
        mappings.

        arg     proxy (osid.proxy.Proxy): a proxy
        return: (osid.repository.CompositionRepositoryAssignmentSession)
                - a CompositionRepositoryAssignmentSession
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented -
                supports_composition_repository_assignment() is false
        compliance: optional - This method must be implemented if
                    supports_composition_repository_assignment() is
                    true.

        """
        if not self.supports_composition_repository_assignment():
            raise Unimplemented()
        try:
            from . import sessions
        except ImportError:
            raise  # OperationFailed()
        proxy = self._convert_proxy(proxy)
        try:
            session = sessions.CompositionRepositoryAssignmentSession(proxy, runtime=self._runtime)
        except AttributeError:
            raise  # OperationFailed()
        return session