code: def pix_to_regular(self):
          
          pix_to_regular = [[] for _ in range(self.pixels)]
  
          for regular_pixel, pix_pixel in enumerate(self.regular_to_pix):
  
              pix_to_regular[pix_pixel].append(regular_pixel)
  
          return pix_to_regular
docs: Compute the mappings between a pixelization's pixels and the unmasked regular-grid pixels. These mappings \
          are determined after the regular-grid is used to determine the pixelization.
  
          The pixelization's pixels map to different number of regular-grid pixels, thus a list of lists is used to \
          represent these mappings