code: def boundaries_lonlat(self, healpix_index, step):
          
          return boundaries_lonlat(healpix_index, step, self.nside, order=self.order)
docs: Return the longitude and latitude of the edges of HEALPix pixels
  
          This returns the longitude and latitude of points along the edge of each
          HEALPIX pixel. The number of points returned for each pixel is ``4 * step``,
          so setting ``step`` to 1 returns just the corners.
  
          Parameters
          ----------
          healpix_index : `~numpy.ndarray`
              1-D array of HEALPix pixels
          step : int
              The number of steps to take along each edge.
  
          Returns
          -------
          lon, lat : :class:`~astropy.units.Quantity`
              The longitude and latitude, as 2-D arrays where the first dimension is
              the same as the ``healpix_index`` input, and the second dimension has
              size ``4 * step``.