code: def find_coverage(self, zoom):
          
          # Find a group of adjacent available tiles at this zoom level
          rows = self._query('''SELECT tile_column, tile_row FROM tiles
                                WHERE zoom_level=?
                                ORDER BY tile_column, tile_row;''', (zoom,))
          t = rows.fetchone()
          xmin, ymin = t
          previous = t
          while t and t[0] - previous[0] <= 1:
              # adjacent, go on
              previous = t
              t = rows.fetchone()
          xmax, ymax = previous
          # Transform (xmin, ymin) (xmax, ymax) to pixels
          S = self.tilesize
          bottomleft = (xmin * S, (ymax + 1) * S)
          topright = ((xmax + 1) * S, ymin * S)
          # Convert center to (lon, lat)
          proj = GoogleProjection(S, [zoom])  # WGS84
          return proj.unproject_pixels(bottomleft, zoom) + proj.unproject_pixels(topright, zoom)
docs: Returns the bounding box (minx, miny, maxx, maxy) of an adjacent
          group of tiles at this zoom level.