olemus
andmestruktuur, mis pärast muutmist
säilitab eelmise versiooni
= a data structure that always preserves the previous version of itself when it is modified

ülevaateid
https://en.wikipedia.org/wiki/Persistent_data_structure

https://arpitbhayani.me/blogs/persistent-data-structures-introduction

https://ocw.mit.edu/courses/6-854j-advanced-algorithms-fall-2005/resources/n2_persistent/