Generalize `merge_ext`.

5 jobs for master in 7 minutes and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
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passed #6102
fp-timing
build-coq.8.6.1

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passed #6101
fp-timing
build-coq.8.7.0

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  Deploy
passed #6103
opam

00:00:13

 
  Build More
passed #6105
fp-timing
build-coq.8.6

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passed #6104
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build-coq.8.7.dev

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