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A Liveness Example in TLA+
If you’ve ever sat at a stop light that was just stuck on red, where there was clearly a problem with the light where it wasn’t ever switching green, you’ve encountered a liveness…
If we check this with the spec from above, the model checker will find a behavior that is permitted by our specification, but that violates this property. If we check this property against our spec with TLC, it quickly finds a counterexample, the scenario where the elevator just sits on the ground floor forever! The model checker can provide us with a counterexample to help explain why this liveness property doesn’t guarantee that the elevator always eventually visits all floors:
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