{- Returns `+1` if the `Optional` value is present and `+0` if the value is absent Examples: ``` ./length Integer ([ 2 ] : Optional Integer) = +1 ./length Integer ([] : Optional Integer) = +0 ``` -} let length : ∀(a : Type) → Optional a → Natural = λ(a : Type) → λ(xs : Optional a) → Optional/fold a xs Natural (λ(_ : a) → +1) +0 in length