Persistent union-find
Implemented by Jason Hemann and Dan Friedman.
Based on a design by Sylvain Conchon and Jean-Christophe Filliâtre.
See A Persistent Union-Find Data Structure. In ACM SIGPLAN Workshop on ML, pages 37-45, Freiburg, Germany, October 2007.
This implementation extends the paper by allowing new elements to be added to the set.