code: def obfn_f(self, X=None):
          r
          if X is None:
              X = self.X
  
          return 0.5 * np.linalg.norm((self.D.dot(X) - self.S).ravel())**2
docs: r"""Compute data fidelity term :math:`(1/2) \| D \mathbf{x} -
          \mathbf{s} \|_2^2`.