def is_sum_of_extended_pdfs(model) -> bool:
    """Checks if the input model is a sum of extended models.

    Args:
         model: the input model/pdf

    Returns:
         True if the model is a sum of extended models, False if not.
    """
    if not hasattr(model, "get_models"):
        return False

    return all(m.is_extended for m in model.get_models())