Explanation: Every ❰List❱ documents the type of its elements with a type
annotation, like this:


    ┌──────────────────────────┐
    │ [1, 2, 3] : List Integer │  A ❰List❱ of three ❰Integer❱s
    └──────────────────────────┘
                       ⇧
                       The type of the ❰List❱'s elements, which are ❰Integer❱s


    ┌───────────────────┐
    │ [] : List Integer │  An empty ❰List❱
    └───────────────────┘
                ⇧
                You still specify the type even when the ❰List❱ is empty


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


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


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


Even if the ❰List❱ is empty you still must specify a valid type

You declared that the ❰List❱'s elements should have type:

↳ $txt0

... which is not a ❰Type❱