But the problem statement doesn't imply the rule involves ordering.
Likely, it doesn't accept complex numbers (or vectors or matrices) as input as part of the (implicit) spec: in common parlance, "number" tends to mean real number.
The tests with real numbers indicate that the rule is one based on order. So the problem doesn't state this but it is easily deducible when working with real numbers. Hence no need to check complex numbers. In common parlance in mathematical circles "number" tends not to mean real number unless context makes this obvious.
