{-
Returns the first non-empty `Optional` value in a `List`

Examples:

```
./head
Integer
(   [[] : Optional Integer, [1] : Optional Integer, [2] : Optional Integer]
    : List (Optional Integer)
)
= [1] : Optional Integer

./head
Integer
([[] : Optional Integer, [] : Optional Integer] : List (Optional Integer))
= [] : Optional Integer

./head Integer ([] : List (Optional Integer))
= [] : Optional Integer
```
-}
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)
        →   Optional/fold a l (Optional a) (λ(x : a) → [x] : Optional a) r
        )
        ([] : Optional a)

in  head