Bogus safety violation checking if a set is a subset of Nat. #6983

Job	Run time
format-check	3m 45s
compiler-warnings	2m 11s
docker-tests	21m 1s
build-mdbook	7s
integration-tests (ubuntu-latest, oopsla19)	19m 0s
scaladoc-check	3m 0s
integration-tests (ubuntu-latest, arrays)	9m 27s
unit-tests (ubuntu-latest)	4m 48s
integration-tests (macos-latest, oopsla19)	17m 7s
unit-tests (macos-latest)	4m 0s
	1h 24m 26s

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