Can you explain to me what the distinction between persistant and ephemeral data structures are (as mentioned in the thesis)? From a brief reading, my understanding is that persistance implies a record of all changes to a data structure are kept somewhere, but that this is only really useful for multithreaded programs. Is that correct?

in a persistent data structure, you don't actively remove, delete, or change anything. rather each time you make a 'change', you create a new structure which points to, and reuses as much of the old one as possible. then the garbage collector removes the unused bits, if any.