Appearance
sliding 方法返回一个新分组的块集合
collect([1, 2, 3, 4, 5]) ->sliding(2); // [[1, 2], [2, 3], [3, 4], [4, 5]]
collect([1, 2, 3, 4, 5]) ->sliding(3, step: 2); // [[1, 2, 3], [3, 4, 5]]