Simplify proof of `list_find_ext`.

8 jobs for master in 8 minutes and 37 seconds
Status Job ID Name Coverage
  Build
passed #36945
fp
build-coq.8.10.dev

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passed #36951
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build-coq.8.7.2

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fp-timing
build-coq.8.8.0

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passed #36949
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build-coq.8.8.1

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passed #36948
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build-coq.8.8.2

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passed #36947
fp-timing
build-coq.8.9.0

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passed #36946
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build-coq.8.9.1

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passed #36944
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build-coq.dev

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