{-
Returns `True` if the `Optional` value is absent and `False` if present
-}
let null
    : ∀(a : Type) → Optional a → Bool
    =   λ(a : Type)
      → λ(xs : Optional a)
      → Optional/fold a xs Bool (λ(_ : a) → False) True

let example0 = assert : null Natural (Some 2) ≡ False

let example1 = assert : null Natural (None Natural) ≡ True

in  null