There is also a bit of weirdness here in that they aren't really the same thing, For example see this persistent union-find which is not immutable, but ensures

There is also a bit of weirdness here in that they aren't really the same thing, For example see this persistent union-find which is not immutable, but ensures its side-effects are safely hidden. https://www.lri.fr/~filliatr/ftp/publis/puf-wml07.pdf https://www.lri.fr/~filliatr/ftp/publis/puf-wml07.pdf