{-
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