def add_validation_method(cls, method):
        """
        Extend validation of Node by adding a function to the _additional_validation list.
        The additional validation function will be called by the clean method

        :method function: function to be added to _additional_validation
        """
        method_name = method.func_name

        # add method name to additional validation method list
        cls._additional_validation.append(method_name)

        # add method to this class
        setattr(cls, method_name, method)
Extend validation of Node by adding a function to the _additional_validation list.
        The additional validation function will be called by the clean method

        :method function: function to be added to _additional_validation