return [ [ 2, 3, 4, 5 ], [ 1, 5, 6, 7 ], [ 1, 7, 8, 9 ], [ 1, 9, 10, 11 ], 
  [ 1, 11, 12, 2 ], [ 2, 12, 13, 14 ], [ 2, 14, 15, 3 ], [ 3, 15, 16, 9 ], 
  [ 3, 8, 10, 4 ], [ 4, 9, 16, 17 ], [ 4, 17, 12, 5 ], [ 5, 11, 18, 6 ], 
  [ 6, 18, 19, 14 ], [ 6, 13, 15, 7 ], [ 7, 14, 19, 8 ], [ 8, 19, 20, 10 ], 
  [ 10, 20, 18, 11 ], [ 12, 17, 20, 13 ], [ 13, 20, 16, 15 ], 
  [ 16, 19, 18, 17 ] ];
