Variants of decidable_exists_nat #3284

Job	Run time
nix	2m 29s
build (supported)	3m 23s
quick-build (supported, ubuntu-latest)	3m 18s
opam-build (supported, ubuntu-latest)	5m 13s
build (latest)	3m 28s
quick-build (latest, ubuntu-latest)	3m 12s
opam-build (latest, ubuntu-latest)	4m 24s
quick-build (dev, ubuntu-latest)	3m 25s
build (dev, --warnings)	3m 30s
opam-build (dev, ubuntu-latest)	7m 11s
doc-timing	1m 47s
doc-alectryon	15m 6s
doc-coqdoc	1m 37s
coqchk (supported)	2m 5s
doc-dep-graphs	7m 41s
coqchk (latest)	2m 0s
install (supported)	1m 35s
coqchk (dev)	1m 50s
install (latest)	1m 31s
install (dev)	1m 50s
deploy-doc	0s
delete-artifacts	10s
	1h 16m 45s

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