code: def common_parent(coords, parent_zoom):
      
      parent = None
      for coord in coords:
          assert parent_zoom <= coord.zoom
          coord_parent = coord.zoomTo(parent_zoom).container()
          if parent is None:
              parent = coord_parent
          else:
              assert parent == coord_parent
      assert parent is not None, 'No coords?'
      return parent
docs: Return the common parent for coords
  
      Also check that all coords do indeed share the same parent coordinate.