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In this example, a thread or fiber enters scope1, schedules a fiber, then enters scope2 where it schedules a fiber in that scope. The execution cannot exit scope2 until the fiber scheduled in that scope terminates. When it exits, it is back in scope1, where it scheduled another fiber. It cannot exit scope1 until the two fibers scheduled in that scope have terminated.

Structured concurrency leads naturally More generally, the structured approach leads to trees of tasks. Consider the following method x that schedules two fibers to execute foo and bar. The code in foo and bar each schedule two fibers.

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fiber1 is scheduled in scope1 to execute foo. It enters scope2 and schedules two fibers. It exits scope2 (and returns to scope1) when the two fibers terminate. fiber2 is scheduled in scope1 to execute bar. It enters scope3 and schedules two fibers. It exits scope3 (and returns to scope1) when the two fibers terminate. The thread or fiber executing will not exit scope1 until fiber1 and fiber2 have terminated.

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