pub fn spawn_default_pooled<T, R, I, Fun, Fut>(
    iter: impl IntoIterator<Item = I>,
    fun: Fun
) -> (ChildPool<R>, Address<T>) where
    Fun: FnOnce(I, Inbox<T>) -> Fut + Send + 'static + Clone,
    Fut: Future<Output = R> + Send + 'static,
    R: Send + 'static,
    T: Send + 'static,
    I: Send + 'static, 
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