template<class T>
struct task {
   struct promise_type {
       auto get_return_object() {
           return task(std::coroutine_handle<promise_type>::from_promise(*this));
       }
       std::suspend_always initial_suspend() { return {}; }
       struct final_awaiter {
           bool await_ready() noexcept { return false; }
           void await_resume() noexcept {}
           std::coroutine_handle<> await_suspend(std::coroutine_handle<promise_type> h) noexcept {
               // final_awaiter::await_suspend is called when the execution of the
               // current coroutine (referred to by 'h') is about to finish.
               // If the current coroutine was resumed by another coroutine via
               // co_await get_task(), a handle to that coroutine has been stored
               // as h.promise().previous. In that case, return the handle to resume
               // the previous coroutine.
               // Otherwise, return noop_coroutine(), whose resumption does nothing.

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