floating point operand stack 浮点操作数堆栈
In the overloaded context they lose the guarantee that the left operand is evaluated before the second and that there is a sequence point inbetween.
因为在重载的上下文中，他们失去了左操作数的赋值在第二个之前的保证，而且之间也会有顺序点。
If the second operand is a Point, the method should return a new Point whose x coordinate is the sum of the x coordinates of the operands, and likewise for they coordinates.
如果第二个运算数是一个Point，该方法就应该返回一个新的Point，新点的横纵坐标分别为两个点坐标相加。
The side effect of updating the stored value of the operand shall occur between the previous and the next sequence point.
而这种“自增”的副作用会在上一个“序列点”跟下一个“序列点”之间完成。
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