Data Structures, Algorithms, & Applications in C++
Chapter 11, Exercise 59

Any sequence which always combines sets that have the same number of elements will do. For example, the sequence union(1,2), union(3,4), union(1,3) results in a tree of 4 elements and height 3. The sequence union(5,6), union(7,8), union(5,7) also results in a height 3 tree with 4 elements. Performing the operation union(1,5) now results in a height 4 tree with 8 elements.