I'm reading for the first time through the docs and the Regions section confused, and I think the example code is wrong. It says
https://doc.flix.dev/regions.html
def toMutDeque(rc: Region[r], l: List[a]): MutDeque[a, rc] \ rc =
let d = MutDeque.empty(rc);
forEach(x -> MutDeque.pushBack(x, d), l);
d
But rc is a value, not a type. I think it should read r.