code: def last_modified(self, name: str = None) -> str:
  		
  		if name is not None:
  			return self[name].last_modified()
  		ts = ""
  		for name in self.keys():
  			if ts is None:
  				ts = self[name].last_modified()
  			else:
  				if self[name].last_modified() > ts:
  					ts = self[name].last_modified()
  		return ts
docs: Return a compact ISO8601 timestamp (UTC timezone) indicating when the layer was last modified
  
  		Note: if name is None, the modification time of the most recently modified layer is returned