@classmethod<EOL><INDENT>def add_validation_method(cls, method):<DEDENT>
method_name = method.func_name<EOL>cls._additional_validation.append(method_name)<EOL>setattr(cls, method_name, method)<EOL>
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