Just curious about the mention of persistent data structures. Is this about the FP notion of persistent data structures (which I suspect) or on-disk data struct

Just curious about the mention of persistent data structures. Is this about the FP notion of persistent data structures (which I suspect) or on-disk data structures? Or both? :) I find that (FP) persistent data structures are like a super-power in lots of ways. (And it only costs a log-N slowdown in the worst case.)