code: def obfn_dfd(self):
          r
  
          Ef = self.eval_Rf(self.Xf)
          return sl.rfl2norm2(Ef, self.S.shape, axis=self.cri.axisN) / 2.0
docs: r"""Compute data fidelity term :math:`(1/2) \| \sum_m
          \mathbf{d}_m * \mathbf{x}_m - \mathbf{s} \|_2^2`.