Suggestion: Operator `in` for slices

[Ola Fosheim Grøstad]

On Monday, 20 December 2021 at 11:04:00 UTC, Dennis wrote:
> On Sunday, 19 December 2021 at 18:31:46 UTC, Steven 
> Schveighoffer wrote:
>> There's your answer -- make a custom type that allows `in` 
>> with an array-like structure.
>
> For the record, I'm not arguing in favor of `in` for slices. 
> I'm just baffled that whenever this comes up, the first thing 
> people bring up is the 'time complexity' argument.

Yes, I agree with this. When I use "in" in Python I am very much 
aware that it does a linear scan, but it is a very useful 
shorthand that can replaces a long and confusing sequence of 
or-expressions.

E.g.

```
if keyword in ['if', 'while', 'for]
```

is a very useful and clear syntactical construct.