<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)