concatMap

{-
Transform 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 ]

./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