docs: def index_pix_in_pixels(pix,pixels,sort=False,outside=-1):
      """
      Find the indices of a set of pixels into another set of pixels.
      !!! ASSUMES SORTED PIXELS !!!
  
      Parameters:
      -----------
      pix    : set of search pixels
      pixels : set of reference pixels
    
      Returns:
      --------
      index : index into the reference pixels
      """
      # ADW: Not really safe to set index = -1 (accesses last entry); 
      # -np.inf would be better, but breaks other code...
  
      # ADW: Are the pixels always sorted? Is there a quick way to check?
      if sort: pixels = np.sort(pixels)
  
      # Assumes that 'pixels' is pre-sorted, otherwise...???
      index = np.searchsorted(pixels,pix)
      if np.isscalar(index):
          if not np.in1d(pix,pixels).any(): index = outside
      else:
          # Find objects that are outside the pixels
          index[~np.in1d(pix,pixels)] = outside
      return index