stack extension

例句：

1. 1. The boxes are stacked neatly on the shelves.
   这些箱子整齐地堆放在架子上。
2. 2. He stacked the books in the corner of the room.
   他把书堆在房间的角落里。
3. 3. The mountain was stacked with snow.
   山上堆满了雪。