This library defines a propositional equality data type,
shims Data.Type.Equality as well as possible for older GHCs (< 7.8).

data a :~: b where
    Refl :: a :~: a

The module Data.Type.Equality.Hetero shims :~~: equality, for compilers
with PolyKinds

Homepage:
http://hackage.haskell.org/package/type-equality
