The `case` form is terrible, and I wish it didn't exist.  I never use
it, and I've only seen new Racketeers use it wrong.  You never want
`case`, you want `cond` or `match` instead.

The `case` form only allows literal data in the match, so if you try
to use a variable it will be interpreted as a symbol instead!
Moreover, `case` and `cond` both have a terrible design mistake that
the fall-through case returns `void` instead of raising an error!
I've only seen `case` result in tiresome debugging.