docs: def _rebuild_indicator(self):
          """ update the indicator """
          indicator = {'term':np.array([]),
                       'row':np.array([]),
                       'col':np.array([])}
  
          for term in range(self.n_terms):
              L = self.A[term].shape[0]
              K = self.F[term].shape[1]
              _update = {'term': (term+1)*np.ones((K,L)).T.ravel(),
                      'row': np.kron(np.arange(K)[:,np.newaxis],np.ones((1,L))).T.ravel(),
                      'col': np.kron(np.ones((K,1)),np.arange(L)[np.newaxis,:]).T.ravel()}
              for key in list(_update.keys()):
                  indicator[key] = np.concatenate([indicator[key],_update[key]])
          self.indicator = indicator