I hit a roadblock trying to understand linear types, because I couldn't figure out what is meant to "use" a variable. I eventually think I figured it out, by reading "Linear types can change the world!", but it was not easy to do so. I think a better description of what "use" means would be very helpful for those unfamiliar with linear types, so I took a stab at it. Hope it helps.
This is mostly to make it easier to write linear function types without
having to invent names for everything, which might be noisy. Also it
improves the display of linear function types when the name isn't used
in the scope.
