{-
`fold` is the primitive function for consuming `List`s

If you treat the list `[ x, y, z ]` as `cons x (cons y (cons z nil))`, then a
`fold` just replaces each `cons` and `nil` with something else

Examples:

```
    ./fold
    Natural
    [ +2, +3, +5 ]
    Natural
    (λ(x : Natural) → λ(y : Natural) → x + y)
    +0
=   +10

    λ(nil : Natural)
→   ./fold
    Natural
    [ +2, +3, +5 ]
    Natural
    (λ(x : Natural) → λ(y : Natural) → x + y)
    nil
=   λ(nil : Natural) → +2 + +3 + +5 + nil

    λ(list : Type)
→   λ(cons : Natural → list → list)
→   λ(nil : list)
→   ./fold Natural [ +2, +3, +5 ] list cons nil
=   λ(list : Type)
→   λ(cons : Natural → list → list)
→   λ(nil : list)
→   cons +2 (cons +3 (cons +5 nil))
```
-}
    let fold
        :   ∀(a : Type)
          → List a
          → ∀(list : Type)
          → ∀(cons : a → list → list)
          → ∀(nil : list)
          → list
        = List/fold

in  fold