Improve mapping out of a generated subgroup #3215

Job	Run time
nix	2m 51s
opam-build (supported, ubuntu-latest)	5m 11s
build (supported)	3m 25s
opam-build (latest, ubuntu-latest)	5m 3s
quick-build (supported, ubuntu-latest)	3m 25s
build (latest)	3m 20s
opam-build (dev, ubuntu-latest)	4m 18s
quick-build (latest, ubuntu-latest)	3m 17s
build (dev, --warnings)	3m 25s
quick-build (dev, ubuntu-latest)	3m 24s
doc-timing	1m 33s
doc-dep-graphs	7m 34s
doc-alectryon	15m 25s
coqchk (supported)	2m 1s
coqchk (latest)	2m 5s
install (supported)	1m 42s
doc-coqdoc	1m 32s
coqchk (dev)	1m 57s
install (latest)	1m 41s
install (dev)	1m 51s
deploy-doc	0s
delete-artifacts	5s
	1h 15m 5s

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