They're saying you should be able to define your own The domain of int/str is infinite so there's less sense in redefining a number system for every property y

They're saying you should be able to define your own The domain of int/str is infinite so there's less sense in redefining a number system for every property you want to model For small finite domains it makes sense to define your own in terms of the problem domain In this sense all two-valued types are bools