1. Bug fix in minElementPair() and maxElementPair(). It is not enough to check that the tree is not empty (this is synonymous to the node being N0).  It could be the case as we traverse down the tree we find N1 -> N0 final nodes.  In this case we also want to consider this an empty tree. Of course such a tree cannot occur at the top level so our definition of isEmpty() is correct -- this is just a different notion of emptiness.
2. Implement functions higherPair() and lowerPair() for the Brother trees -- they make use of the functions above and that's how the bug was found.
3. Add one test for those functions.