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Explanation: The syntax for ❰Optional❱ values resembles the syntax for ❰List❱s:
┌───────────────────────┐
│ [] : Optional Integer │ An ❰Optional❱ value which is absent
└───────────────────────┘
┌───────────────────────┐
│ [] : List Integer │ An empty (0-element) ❰List❱
└───────────────────────┘
┌────────────────────────┐
│ [1] : Optional Integer │ An ❰Optional❱ value which is present
└────────────────────────┘
┌────────────────────────┐
│ [1] : List Integer │ A singleton (1-element) ❰List❱
└────────────────────────┘
However, an ❰Optional❱ value can $_NOT have more than one element, whereas a
❰List❱ can have multiple elements:
┌───────────────────────────┐
│ [1, 2] : Optional Integer │ Invalid: multiple elements $_NOT allowed
└───────────────────────────┘
┌───────────────────────────┐
│ [1, 2] : List Integer │ Valid: multiple elements allowed
└───────────────────────────┘
Your ❰Optional❱ value had this many elements:
↳ $txt0
... when an ❰Optional❱ value can only have at most one element
Some common reasons why you might get this error:
● You accidentally typed ❰Optional❱ when you meant ❰List❱, like this:
┌────────────────────────────────────────────────────┐
│ List/length Integer ([1, 2, 3] : Optional Integer) │
└────────────────────────────────────────────────────┘
⇧
This should be ❰List❱ instead
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