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