More uses of operator "in"

["Nordlöw" via Digitalmars-d-learn]

On Tuesday, 28 October 2014 at 18:29:49 UTC, Marc Schütz wrote:
> It always was the argument against generalizing `in` to arrays. 
> `in` is not alone in this respect. It's also expected that 
> indexing and slicing be "cheap" operations.

BTW: Does anybody have an efficient unordered set container lying 
around somewhere?