docs: def bi_square(xx, idx=None):
      """
      The bi-square weight function calculated over values of xx
  
      Parameters
      ----------
      xx: float array
  
      Notes
      -----
      This is the first equation on page 831 of [Cleveland79].
      """
  
      ans = np.zeros(xx.shape)
      ans[idx] = (1-xx[idx]**2)**2
      return ans