Let's start with the back side, which will be represented by two diagonal lines, across the CardElement card. page 
I am a (model of a) Card in the Memory Game.
Two diagonal lines, across the CardElement Card
> I have a front and a **Back Side**. You can flip me to show the other side. If I have been matched I disappear and am no longer visible. [⇒ Cheshire Cat]
We introduce a new `back` instance variable, that will be initialized in the CardElement>>#initialize:
initialize super initialize. self size: self cardSize. self background: self backgroundColor. self geometry: (BlRoundedRectangleGeometry cornerRadius: 12). self initializeBack. self addChild: back.
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DOT FROM two-level-diagram
BrCardExamples>>#verticalCard