Note [Bottom as a usage]

What is the usage of x in

   case … of {}

Per usual linear logic, as well as the _Linear Haskell_ article, x can have
every multiplicity.

So we need a minimum usage _bottom_, which is also the neutral element for join.

In fact, this is not such as nice solution, because it is not clear how to
define sum and multiplication with bottom. We give reasonable definitions, but
they are not complete (they don't respect the semiring laws, and it's possible
to come up with examples of Core transformation which are not well-typed)

A better solution would probably be to annotate case expressions with a usage
environment, just like they are annotated with a type. Which, probably not
coincidentally, is also primarily for empty cases.

A side benefit of this approach is that the linter would not need to join
multiplicities, anymore; hence would be closer to the presentation in the
article. That's because it could use the annotation as the multiplicity for each
branch.