code: def _neighbour_pixels(hp, ipix):
          
          neigh_ipix = np.unique(hp.neighbours(ipix).ravel())
          # Remove negative pixel values returned by `~astropy_healpix.HEALPix.neighbours`
          return neigh_ipix[np.where(neigh_ipix >= 0)]
docs: Returns all the pixels neighbours of ``ipix``