code: def display(self, depth=0):
          
          print(depth * "    " + "[level %s]" % self.level)
          for item in self.items:
              if isinstance(item, Cluster):
                  item.display(depth + 1)
              else:
                  print(depth * "    " + "%s" % item)
docs: Pretty-prints this cluster. Useful for debuging.