code: def cnst_A(self, X, Xf=None):
          r
  
          if Xf is None:
              Xf = sl.rfftn(X, axes=self.axes)
          return sl.irfftn(self.GAf*Xf[..., np.newaxis], self.axsz,
                           axes=self.axes)
docs: 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}`.