{- Flatten two `Optional` layers into a single `Optional` layer Examples: ``` ./concat Integer ([[1] : Optional Integer] : Optional (Optional Integer)) = [1] : Optional Integer ./concat Integer ([[] : Optional Integer] : Optional (Optional Integer)) = [] : Optional Integer ./concat Integer ([] : Optional (Optional Integer)) = [] : Optional Integer ``` -} let concat : ∀(a : Type) → Optional (Optional a) → Optional a = λ(a : Type) → λ(x : Optional (Optional a)) → Optional/fold (Optional a) x (Optional a) (λ(y : Optional a) → y) ([] : Optional a) in concat