Opaque types like "Integer" don't give a lot of assurance, so either you get a stronger type system (Agda-style) or you have to test. But complicated code may

Opaque types like "Integer" don't give a lot of assurance, so either you get a stronger type system (Agda-style) or you have to test. But complicated code may work with non-opaque types, and so it could still be reliable without tests.