code: def obfn_f(self):
          r
  
          obf = 0.0
          for i in range(self.Nb):
              obf += self.obfn_fi(self.obfn_fvar(i), i)
          return obf
docs: r"""Compute :math:`f(\mathbf{x}) = \sum_i f(\mathbf{x}_i)`
          component of ADMM objective function.