{-
Returns the first non-empty `Optional` value in a `List`
-}
let head
    : ∀(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)
            → merge { Some = λ(x : a) → Some x, None = r } l
          )
          (None a)

let example0 = assert : head Natural [ None Natural, Some 1, Some 2 ] ≡ Some 1

let example1 =
      assert : head Natural [ None Natural, None Natural ] ≡ None Natural

let example2 =
      assert : head Natural ([] : List (Optional Natural)) ≡ None Natural

in  head