4.21: Handling the First Clicked Box

                if firstSelection == None: # the current box was the first box clicked
                    firstSelection = (boxx, boxy)
                else: # the current box was the second box clicked
                    # Check if there is a match between the two icons.
                    icon1shape, icon1color = getShapeAndColor(mainBoard, firstSelection[0], firstSelection[1])
                    icon2shape, icon2color = getShapeAndColor(mainBoard, boxx, boxy)

Before the execution entered the game loop, the firstSelection variable was set to None. Our program will interpret this to mean that no boxes have been clicked, so if line 1's [90] condition is True, that means this is the first of the two possibly matching boxes that was clicked. We want to play the reveal animation for the box and then keep that box uncovered. We also set the firstSelection variable to a tuple of the box coordinates for the box that was clicked.

If this is the second box the player has clicked on, we want to play the reveal animation for that box but then check if the two icons under the boxes are matching. The getShapeAndColor() function (explained later) will retrieve the shape and color values of the icons. (These values will be one of the values in the ALLCOLORS and ALLSHAPES tuples.)

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