def validate_parent(parent)
        raise InvalidParent.create(parent, self) unless (!parent.nil? && is_valid_parent?(parent))
        parent.validate_child(self)
      end
def validate_parent(parent)
        raise InvalidParent.create(parent, self) unless (!parent.nil? && is_valid_parent?(parent))
        parent.validate_child(self)
      end
determine whether the parent is valid for this object. See description of
 validate_child for more detail. This method calls validate_child to perform
 the actual validation.
 @param [Taxonomite::Node] parent the parent to validate
 @return [Boolean] whether validation was successful