code: def _kendall_tau_add(self, len_old, diff_pos, tau_old):
          
          return 2./(len_old+1)*(float(diff_pos)/len_old-tau_old)
docs: Compute Kendall tau delta.
  
          The new sequence has length len_old + 1.
  
          Parameters
          ----------
          len_old : int
              The length of the old sequence, used to compute tau_old.
          diff_pos : int
              Difference between concordant and non-concordant pairs.
          tau_old : float
              Kendall rank correlation of the old sequence.