docs: 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)