我们要来定义一个带有带有高和宽的矩形，并且要提供一个计算其面积的area方法。
We want to write a class Rectangle, which has width and height as its data and a method area that computes the area.
这个矩形的尺寸应该是250 px宽50 px高。
The dimensions of this shape should be 250px wide and 50px high.
您在配料清单中看到的row类将设置42像素高的块，该块适用于圆角矩形内的一叠行中的一行。
The row class, seen in the ingredient listing, sets a 42-pixel high block, suitable for a single line in a stack of lines inside a rounded rectangle.
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