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DeBruijn: refresh a function description given its new implementation (#542)

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Marko Dimjašević 2020-10-24 17:00:27 +02:00 committed by GitHub
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src/plfa/part2/DeBruijn.lagda.md
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@ -428,10 +428,8 @@ lookup {(_ , A)} {zero} (s≤s z≤n) = A
lookup {(Γ , _)} {(suc n)} (s≤s p) = lookup p
```
We intend to apply the function only when the natural is
shorter than the length of the context, which we indicate by
postulating an `impossible` term, just as we did
[here](/Lambda/#primed).
We intend to apply the function only when the natural is shorter than
the length of the context, which is witnessed by `p`.
Given the above, we can convert a natural to a corresponding
de Bruijn index, looking up its type in the context: