by y itself 自动地 ; 独自地
The Panel itself has x and y coordinates.
Panel 本身具有 x 和 y 坐标。
That element is itself a list, so the second indexing ([y]) returns one of the nested list's elements.
该元素本身也是一个列表,因此第二个索引操作([y])返回嵌套列表的元素之一。
At every instant, it's got a location given by the vector R; R itself is contained in a pair of numbers, x and y, and they vary with time.
在每一个瞬时,它的位置由位矢 R 给出,R 本身包含了一对坐标值 x 和 y,并且它们都随着时间的变化而变化
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