What's the purpose of the 'in' keyword ?

[Adam D. Ruppe]

On Sunday, 27 May 2018 at 13:02:23 UTC, loloof64 wrote:
> What's the purpose of this 'in' keyword ? I could not process a 
> good Google request to get an answer.

It means you are taking the parameter in to look at, but not 
modify or store.

Basically "const". (well, for now, literally "const" but that can 
change with other details)