## Permit abstract data types in signatures that don't have kind *

Here is a fascinating program that I would like to write, but cannot with Backpack today:

```
unit p where
signature Key where
import GHC.Exts (Constraint)
data Key k :: Constraint
instance Key Bool
signature Map where
import Key
data Map k a
empty :: Map k a
insert :: Key k => k -> a -> Map k a -> Map k a
lookup :: Key k => k -> Map k a -> Maybe a
module M where
import Map
import Key
x = insert True "foo" empty
```

The point is to make the *constraint* for Map parametrizable, so that HashMap can levy a different constraint (Hashable) than Map (Ord).

`data Key k :: Constraint`

is implementable, because we can implement abstract data types using type synonyms in Backpack, and synonyms can have more kinds than just `*`

. However, GHC chokes on this declaration, because it thinks that data types always have to have kind `*`

.