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

  let Number/add = ./add.dhall nix

  let fromNatural = ./fromNatural.dhall nix

  let sum
      : List Number → Number
      = λ(xs : List Number) →
          List/fold Number xs Number Number/add (fromNatural 0)

  in  sum