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``

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)