I don't know what you're talking about, but nowhere is it documented that in can be used in conjunction with to to perform a range check (which, might I add, is hardly "improper" as it acts exactly the same). It's use in switch is not in conjunction with in and would not nescesarily have to work elsewhere. In addition to that, it's use in the to-step style loops is obviously different, as in that case it's generating a list of integers.
So, please, stop babbling about it being documented when it very clearly is not.
I clearly said this syntax in switch() was documented, which it is. Then I said it's also the most important usage of this syntax to know, as the others are simply shortcuts. I wonder if you can still read.
So, please, stop babbling about it being documented when it very clearly is not.