code: def common_parent(a, b):
      
  
      if a.zoom < b.zoom:
          b = b.zoomTo(a.zoom).container()
  
      elif a.zoom > b.zoom:
          a = a.zoomTo(b.zoom).container()
  
      while a.row != b.row or a.column != b.column:
          a = a.zoomBy(-1).container()
          b = b.zoomBy(-1).container()
  
      # by this point a == b.
      return a
docs: Find the common parent tile of both a and b. The common parent is the tile
      at the highest zoom which both a and b can be transformed into by lowering
      their zoom levels.