def cnst_A0(self, X, Xf=None):
        r"""Compute :math:`A_0 \mathbf{x}` component of ADMM problem
        constraint.
        """

        # This calculation involves non-negligible computational cost
        # when Xf is None (i.e. the function is not being applied to
        # self.X).
        if Xf is None:
            Xf = sl.rfftn(X, None, self.cri.axisN)
        return sl.irfftn(sl.inner(self.Df, Xf, axis=self.cri.axisM),
                         self.cri.Nv, self.cri.axisN)
r"""Compute :math:`A_0 \mathbf{x}` component of ADMM problem
        constraint.