This is a typeclass to represent things which are countable down. Note that it is important
that a value prev(t) is always less than t. Note
that prev returns Option because this class comes with the notion that some items may reach a minimum
key, which is None.
This is a typeclass to represent things which are countable down. Note that it is important that a value prev(t) is always less than t. Note that prev returns Option because this class comes with the notion that some items may reach a minimum key, which is None.