code: def obfn_dfd(self):
          r
  
          Ef = sl.inner(self.Zf, self.obfn_fvarf(), axis=self.cri.axisM) \
            - self.Sf
          return (np.linalg.norm(self.W * sl.irfftn(Ef, self.cri.Nv,
                                                    self.cri.axisN))**2) / 2.0
docs: r"""Compute data fidelity term :math:`(1/2) \| W \left( \sum_m
          \mathbf{d}_m * \mathbf{x}_m - \mathbf{s} \right) \|_2^2`.