Variants of decidable_exists_nat #3289

Job	Run time
quick-build (supported, ubuntu-latest)	3m 16s
nix	2m 32s
quick-build (latest, ubuntu-latest)	3m 17s
opam-build (supported, ubuntu-latest)	5m 11s
opam-build (latest, ubuntu-latest)	4m 25s
quick-build (dev, ubuntu-latest)	3m 18s
build (supported)	3m 21s
opam-build (dev, ubuntu-latest)	7m 36s
build (latest)	3m 30s
build (dev, --warnings)	3m 32s
doc-coqdoc	1m 42s
doc-alectryon	16m 24s
doc-dep-graphs	7m 45s
coqchk (supported)	2m 3s
doc-timing	1m 35s
coqchk (latest)	1m 56s
install (supported)	1m 35s
coqchk (dev)	1m 54s
install (latest)	1m 43s
install (dev)	1m 43s
deploy-doc	0s
delete-artifacts	5s
	1h 18m 23s

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