Explanation: Every optional element ends with a type annotation for the element
that might be present, like this:


    ┌────────────────────────┐
    │ [1] : Optional Integer │  An optional element that's present
    └────────────────────────┘
                     ⇧
                     The type of the ❰Optional❱ element, which is an ❰Integer❱


    ┌────────────────────────┐
    │ [] : Optional Integer  │  An optional element that's absent
    └────────────────────────┘
                    ⇧
                    You still specify the type even when the element is absent


The element type must be a type and not something else.  For example, the
following element types are $_NOT valid:


    ┌──────────────────┐
    │ ... : Optional 1 │
    └──────────────────┘
                     ⇧
                     This is an ❰Integer❱ and not a ❰Type❱


    ┌─────────────────────┐
    │ ... : Optional Type │
    └─────────────────────┘
                     ⇧
                     This is a ❰Kind❱ and not a ❰Type❱


Even if the element is absent you still must specify a valid type

You declared that the ❰Optional❱ element should have type:

↳ $txt0

... which is not a ❰Type❱