To add more detail, I seem to remember, that "persistent" in that book means the following: Once you have a handle on the data structure, you can perform any p

To add more detail, I seem to remember, that "persistent" in that book means the following: Once you have a handle on the data structure, you can perform any pure operation, which is intended for that data structure and your handle will still point to the same data structure, because "modified" versions are not really "modified" as in someone performing a mutation on the data structure, but actually new instances of the data structure, which might share parts with the original one. This property is very nice to have for working with the data structure.