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