The Correctness of Launchbury's Natural Semantics for Lazy Evaluation

In his seminal paper “A Natural Semantics for Lazy Evaluation”, John Launchbury proves his semantics correct with respect to a denotational semantics. We machine-checked the proof and found it to fail, and provide two ways to fix it: One by taking a detour via a modified natural semantics with an explicit stack, and one by adjusting the denotational semantics of heaps.