code: def cnst_AT(self, X):
          r
  
          Xf = sl.rfftn(X, axes=self.axes)
          return np.sum(sl.irfftn(np.conj(self.GAf)*Xf, self.axsz,
                                  axes=self.axes), axis=self.Y.ndim-1)
docs: r"""Compute :math:`A^T \mathbf{x}` where :math:`A \mathbf{x}` is
          a component of ADMM problem constraint. In this case
          :math:`A^T \mathbf{x} = (G_r^T \;\; G_c^T \;\; H^T) \mathbf{x}`.