These kinds of wrapper collections typically aren't mutable – for example, when you called reversed() on an array, the elements of the resulting ReversedCollection aren't mutable, even if your original array is.

That said, I don't think it's necessarily the wrong thing to do here – in general the first mutation will make a copy of whichever base collection ends up getting modified, and from then it will be singly referenced. I've started a discussion in the forums to talk about the question – please add any more thoughts you have: https://forums.swift.org/t/making-collection-wrappers-mutable/69142