fold

{-
`fold` is the primitive function for consuming `Natural` numbers

If you treat the number `+3` as `succ (succ (succ zero))` then a `fold` just
replaces each `succ` and `zero` with something else

Examples:

```
./fold +3 Natural (λ(x : Natural) → +5 * x) +1 = +125

λ(zero : Natural) → ./fold +3 Natural (λ(x : Natural) → +5 * x) zero
= λ(zero : Natural) → +5 * +5 * +5 * zero

    λ(natural : Type)
→   λ(succ : natural → natural)
→   λ(zero : natural)
→   ./fold +3 natural succ zero
=   λ(natural : Type)
→   λ(succ : natural → natural)
→   λ(zero : natural)
→   succ (succ (succ zero))
```
-}
    let fold
        :   Natural
          → ∀(natural : Type)
          → ∀(succ : natural → natural)
          → ∀(zero : natural)
          → natural
        = Natural/fold

in  fold