def cnst_A(self, X, Xf=None):
        r"""Compute :math:`A \mathbf{x}` component of ADMM problem
        constraint.  In this case :math:`A \mathbf{x} = (G_r^T \;\;
        G_c^T \;\; H)^T \mathbf{x}`.
        """

        if Xf is None:
            Xf = sl.rfftn(X, axes=self.axes)
        return sl.irfftn(self.GAf*Xf[..., np.newaxis], self.axsz,
                         axes=self.axes)