docs: def add_behavior_for_pclass(self, definition, cls):
          """
          Define an additional base class for the Python class created for a
          particular definition.
  
          :param unicode definition: The definition the Python class for which
              the base class will be included.
  
          :param type cls: The additional base class.
  
          :raise ValueError: If a Python class for the given definition has
              already been created.  Behavior cannot be retroactively added to a
              Python class.  All behaviors must be registered before the first
              call to ``pclass_for_definition`` for a particular definition.
  
          :return: ``None``
          """
          if definition in self._pclasses:
              raise AlreadyCreatedClass(definition)
          if definition not in self.definitions:
              raise NoSuchDefinition(definition)
          self._behaviors.setdefault(definition, []).append(cls)