def get_asset_composition_session(self):
        """Gets the session for retrieving asset compositions.

        return: (osid.repository.AssetCompositionSession) - an
                AssetCompositionSession
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - supports_asset_composition() is false
        compliance: optional - This method must be implemented if
                    supports_asset_composition() is true.

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