Implement suggestions by Robbert Krebbers on gmap.

Status Job ID Name Coverage
  Build
canceled #39338
fp
build-coq.8.10.dev

canceled #39344
fp
build-coq.8.7.2

canceled #39343
fp-timing
build-coq.8.8.0

canceled #39342
fp
build-coq.8.8.1

canceled #39341
fp
build-coq.8.8.2

canceled #39340
fp-timing
build-coq.8.9.0

canceled #39339
fp
build-coq.8.9.1

canceled #39337
fp
build-coq.dev