Thanks for the pointers to the two existing issues. Though I’d consider !in strictly inferior to not in in terms of readability.
Also thanks to pointing out ∉ – I’ll make sure to use that for demos but I don’t feel comfortable using unicode like that in my actual code (of course that’s a “me” problem 
).
Totally off-topic, but: for iteration, I don’t view in as syntactic sugar for =, but rather = as a horrible alternative syntax for in that is there to appease people coming from certain programming languages – it is IMHO utterly confusing for anyone else :-).