This is output from running AVLTreeTester. In each triple, the first number is
the key, the second number height, and the third number balance. The last tree, obtained 
by inserting 75, is not AVL. The search path is (20, 25, 55, 75). The pivot is 25. The 
AVL property would be restored by performing a single left rotation at 25. Currently, 
this is not implemented and the insert method just quits as soon as the pivot is 
detected.  In fact, this is the reason why node 20 does not have its height and balance 
set correctly - we never get to 20.


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                                        20 0 0                                                                              
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                                        20 1 -1                                                                              
                    15 0 0                                      --                                      
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                                        20 1 0                                                                              
                    15 0 0                                      25 0 0                                      
..............................................................................
..............................................................................
                                        20 2 1                                                                              
                    15 0 0                                      25 1 1                                      
          --                  --                  --                  55 0 0                  
..............................................................................
Tree is no longer AVL.
..............................................................................
                                        20 2 1                                                                              
                    15 0 0                                      25 2 2                                      
          --                  --                  --                  55 1 1                  
     --        --        --        --        --        --        --        75 0 0        
..............................................................................