{-
Add all the numbers in a `List`

Examples:

```
./sum [ +2, +3, +5 ] = +10

./sum ([] : List Natural) = +0
```
-}
    let sum
        : List Natural → Natural
        =   λ(xs : List Natural)
          → List/fold
            Natural
            xs
            Natural
            (λ(l : Natural) → λ(r : Natural) → l + r)
            +0

in  sum