λ(nix : ../NixPrelude.dhall) →
  let fold = ./fold.dhall nix

  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) →
                fold a l (Optional a) (λ(v : a) → Some v) r
            )
            (None a)

  in  head