(doc Phantom "is a type without inhabitants and a free type variable, which is
a fancy way of saying that it has no runtime representation.

When trying to mark types programmatically using something like `the` but
without the need for a special form, `Phantom` comes in handy.

Example:

```
(defn test [a]
  (Phantom.ensure a (the (Phantom Int) (Phantom.init))))

(println* (test 123l)) ; this will not compile
```")
(deftype (Phantom a) [])

(defmodule Phantom
  (doc ensure "is a function that takes anything and a `Phantom`, ensuring that
the type contained by the `Phantom` and the type of the first parameter match.")
  (sig ensure (Fn [a (Phantom a)] a))
  (defn ensure [a p] a)
)