Implementation of our command line parsing combinators

But the above is not a valid Haskell 98 data type!

• Existential quantification is needed for the type of Ap.
• Indexed families of types (aka GADTs) are needed for the type of Many

  • data P a where
        ...
        Ap    :: P (b->a) -> P b -> P a
        ...
        Many  :: P a -> P [a]
        ...