docs: def unregister_class(alias):
      """
      Opposite of L{register_class}.
  
      @raise UnknownClassAlias: Unknown alias.
      """
      try:
          x = CLASS_CACHE[alias]
      except KeyError:
          raise UnknownClassAlias('Unknown alias %r' % (alias,))
  
      if not x.anonymous:
          del CLASS_CACHE[x.alias]
  
      del CLASS_CACHE[x.klass]
  
      return x