docs: def tri_cube(xx, idx):
      """ 
      The tri-cube kernel estimated for xx values at indices idx (zero
      elsewhere) 
  
      Parameters
      ----------
      xx: float array
          Values of the function on which the kernel is computed. Typically,
          these are Euclidean distances from some point x0 (see do_kernel)
  
      idx: tuple
          An indexing tuple pointing to the coordinates in xx for which the
          kernel value is estimated. Default: None (all points are used!)  
  
      Notes
      -----
      This is equation 6.6 in FHT chapter 6        
      """        
  
      ans = np.zeros(xx.shape)
      ans[idx] = (1-np.abs(xx[idx])**3)**3
      return ans