Suggestion: Operator `in` for slices

[Elronnd]

On Saturday, 1 January 2022 at 16:40:02 UTC, Nick Treleaven wrote:
> On Wednesday, 22 December 2021 at 12:43:26 UTC, Ola Fosheim 
> Grøstad wrote:
>> 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.
>
> `in` is defined for RedBlackTree, but it returns bool:
> https://dlang.org/phobos/std_container_rbtree.html#.RedBlackTree.opBinaryRight

A redblacktree is not a sequence, though; its elements are not 
keyed in any way.