code: def obfn_dfd(self):
          r
  
          Ef = self.eval_Rf(self.Xf)
          E = sl.irfftn(Ef, self.cri.Nv, self.cri.axisN)
  
          return (np.linalg.norm(self.W * E)**2) / 2.0
docs: r"""Compute data fidelity term :math:`(1/2) \sum_k \| W (\sum_m
          \mathbf{d}_m * \mathbf{x}_{k,m} - \mathbf{s}_k) \|_2^2`