some helpful lemmas for FreeGroup #3218

Job	Run time
opam-build (supported, ubuntu-latest)	5m 12s
build (supported)	3m 27s
opam-build (latest, ubuntu-latest)	5m 6s
build (latest)	3m 20s
opam-build (dev, ubuntu-latest)	4m 18s
build (dev, --warnings)	3m 23s
nix	2m 30s
quick-build (supported, ubuntu-latest)	3m 13s
quick-build (latest, ubuntu-latest)	3m 26s
quick-build (dev, ubuntu-latest)	3m 14s
doc-timing	1m 41s
doc-dep-graphs	7m 30s
doc-alectryon	15m 10s
coqchk (supported)	2m 2s
doc-coqdoc	1m 29s
install (supported)	1m 45s
coqchk (latest)	1m 59s
coqchk (dev)	1m 59s
install (latest)	1m 34s
install (dev)	1m 55s
deploy-doc	0s
delete-artifacts	10s
	1h 14m 23s

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