code: def obfn_dfd(self):
          r
  
          Ef = sl.inner(self.Df, self.obfn_fvarf(), axis=self.cri.axisM) - \
              self.Sf
          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`.