## Data.Set is not guaranteed to be balanced

"Balanced" means that for each node in the tree, the difference between the depth of the left subtree and that of the right subtree is at most 1. This is not always the case, as can be seen from the following counterexample:

```
$ testproperties -r -T 'Tests 100000' -m Data.Set -C -h -C 200m
...
"union: correctness.balanced" Counterexample 1 found after 97684 tests: ['c','b','A'] ['B',' ','a']
```