Restore a Context Variable with Its Token After a Nested Override

<p>A helper temporarily labels some work, calls another helper, and then needs to restore its caller's label. Assigning a default string on exit is insufficient when the caller already had a different binding. Restoration must preserve the actual earlier state.</p> <p>ContextVar.set returns a token for that purpose. Pair it with reset in a finally block, so normal return and ordinary exceptions bo

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