code: def make_metatiles(size, tiles, date_time=None):
      
  
      groups = defaultdict(list)
      for tile in tiles:
          key = tile['layer']
          groups[key].append(tile)
  
      metatiles = []
      for group in groups.itervalues():
          parent = _parent_tile(t['coord'] for t in group)
          metatiles.extend(make_multi_metatile(parent, group, date_time))
  
      return metatiles
docs: Group by layers, and make metatiles out of all the tiles which share those
      properties relative to the "top level" tile which is parent of them all.
      Provide a 6-tuple date_time to set the timestamp on each tile within the
      metatile, or leave it as None to use the current time.