The "_" at the end shows that this type variable is used only in this place.
Our types have a lot of type variables, most of them only used once. If you see a -_ you know not to focus on these.
See the rule single-use-type-vars-end-with-underscore.
If you have questions, don't hesitate to ask (e.g. in slack (@lue lue.the.bird@gmail.com) or by starting a discussion in github)!
Can't we define Infinity so that it unifies with all other Up types?
type alias Infinity =
Add1 Infinity
but: elm doesn't like recursive aliases and rightfully so here
because such a value is Never constructable.