Suggestion: Operator `in` for slices

[Ola Fosheim Grøstad]

On Tuesday, 21 December 2021 at 17:06:04 UTC, Nick Treleaven 
wrote:
> Dennis is right that conceptually a key for an array is an 
> index. An array element value is the data associated with an 
> index.

What it is conceptually, depends on how you conceptualize it. The 
most common conceptualization of "in" is as a set operation and 
you cannot represent a set with array indexes.