{-
Returns the last non-empty `Optional` value in a `List`

Examples:

```
./last Natural [ None Natural, Some 1, Some 2 ] = Some 2

./last Natural [ None Natural, None Natural ] = None Natural

./last Natural ([] : List (Optional Natural)) = None Natural
```
-}
    let last
        : ∀(a : Type) → List (Optional a) → Optional a
        =   λ(a : Type)
          → λ(xs : List (Optional a))
          → List/fold
            (Optional a)
            xs
            (Optional a)
            (   λ(l : Optional a)
              → λ(r : Optional a)
              → Optional/fold a r (Optional a) (λ(x : a) → Some x) l
            )
            (None a)

in  last