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