Type systems are tools for modeling some aspect of reality. Some types need to represent one of several different choices.Sometimes, all the choices may be known in advance and will likely never change—this is often called a closed universe of values.Other times, the set of options will change over time, and the type needs to represent an open universe.How should these different kinds of choices be modeled?Aside: this matters more for libraries than for applications#If you’re writing an end-user...