immutability

data structures

1. partially persistent data structure
2. full persistent data strucutres
3. persistent immutable data structures
   1. git for datastructures?

efficient immutable data

1. trees and sharing
   1. sharing the structure of the three between two version
2. path copying
3. turning linear into some algorithmic
4. thumb rule
   1. what didn't change can be shared
   2. what changed has to stored separately for each version