True. Well usually. In some cases the property you're writing is so simple you wouldn't need any tests, for example in a compression library you just check deco

True. Well usually. In some cases the property you're writing is so simple you wouldn't need any tests, for example in a compression library you just check decompress(compress(x)) == x. If that is proven there's no need for tests. But yeah usually it's a good idea to have a few tests anyway. You just need fewer tests the more properties you can prove formally.