change Lemma to Definition #1968

Job	Run time
nix	2m 10s
opam-build (supported, ubuntu-latest)	4m 59s
build (supported)	3m 32s
quick-build (supported, ubuntu-latest)	3m 14s
opam-build (latest, ubuntu-latest)	4m 42s
quick-build (latest, ubuntu-latest)	3m 16s
opam-build (dev, ubuntu-latest)	2m 9s
build (latest)	3m 23s
quick-build (dev, ubuntu-latest)	1m 21s
build (dev, --warnings)	1m 32s
doc-dep-graphs	0s
doc-timing	0s
doc-alectryon	0s
coqchk	0s
doc-coqdoc	0s
install	0s
deploy-doc	0s
delete-artifacts	6s
	30m 24s

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