^   t                                                                         
  |   t                                               t                         
  |   t  t  t                                      t  t  t  t  t     t  t       
  |   t  t  t                                      t  t  t  t  t  t  t  t       
  |   t  t  t  t                             t     t  t  t  t  t  t  t  t       
  |   t  t  t  t  t                          t     t  t  t  t  t  t  t  t  t    
  |   t  t  t  t  t  t                    t  t  t  t  t  t  t  t  t  t  t  t    
  |   t  t  t  t  t  t  t                 t  t  t  t  t rt  t  t  t  t  t  t    
  |   t  t  t  t  t rt rt  t  t        t  t  t rt  t  t rt  t  t  t  t  t  t    
  |   t  t  t  t  t rt rt rt  t rt  t rt  t  t rt  t  t rt  t  t  t  t  t rt    
 -+---------------------------------------------------------------------------> 
  |  19 20 21 22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18