It seems like this is proposing syntactic sugar to make mutating and non-mutating operations be on equal footing.

> The more interesting example is reassigning the deeply nested l to make the cat inside older, without mutating the original cat

Isn't that mutating l, though? If you're concerned about mutating cat, shouldn't you be concerned about mutating l?

It doesn't mutate l exactly, it makes a new list slightly different from the original one and assigns it to l.

That means if someone has a reference to the original l, they do not see the change (because l is immutable. Both of them).