code: def add_child_ref(self, transform_ref):
          
          if not isinstance(transform_ref, TransformRef):
              raise ValueError("transform_ref must be of type TransformRef")
  
          for child in self.children:
              if child is transform_ref:
                  return
  
          transform_ref.parent = self
          self.children.append(transform_ref)
docs: Adds the TransformRef if it is not already added.
          :param transform_ref:
          :return: