concatMap

{-
Tranform a list by applying a function to each element and flattening the
results

Examples:

```
./concatMap Natural Natural (λ(n : Natural) → [n, n]) [+2, +3, +5]
= [+2, +2, +3, +3, +5, +5] : List Natural

./concatMap Natural Natural (λ(n : Natural) → [n, n]) ([] : List Natural)
= [] : List Natural
```
-}
    let concatMap
        : ∀(a : Type) → ∀(b : Type) → (a → List b) → List a → List b
        =   λ(a : Type)
          → λ(b : Type)
          → λ(f : a → List b)
          → λ(xs : List a)
          → List/build
            b
            (   λ(list : Type)
              → λ(cons : b → list → list)
              → List/fold a xs list (λ(x : a) → List/fold b (f x) list cons)
            )

in  concatMap