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