code: def obfn_dfd(self):
          r
  
          DXBf = sl.dot(self.B, sl.inner(self.Df, self.obfn_fvarf(),
                                         axis=self.cri.axisM),
                         axis=self.cri.axisC)
          Ef = DXBf - self.Sf
          return sl.rfl2norm2(Ef, self.S.shape, axis=self.cri.axisN) / 2.0
docs: r"""Compute data fidelity term :math:`(1/2) \| D X B - S \|_2^2`.