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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❱
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