def rsdl_rn(self, AX, Y):
        """Compute primal residual normalisation term.

        Overriding this method is required if methods :meth:`cnst_A`,
        :meth:`cnst_AT`, :meth:`cnst_B`, and :meth:`cnst_c` are not
        overridden.
        """

        if not hasattr(self, '_cnst_nrm_c'):
            self._cnst_nrm_c = np.sqrt(np.linalg.norm(self.cnst_c0())**2 +
                                       np.linalg.norm(self.cnst_c1())**2)
        return max((np.linalg.norm(AX), np.linalg.norm(Y), self._cnst_nrm_c))