{-
Flatten two `Optional` layers into a single `Optional` layer

Examples:

```
./concat Natural (Some (Some 1))
= Some 1

./concat Natural (Some (None Natural))
= None Natural

./concat Natural (None (Optional Natural))
= None Natural
```
-}
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)
        (None a)

in  concat