how to get the corners of 2 points on a matrix

1say you have 2 points, (x1,y1) and (x2,y2)
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3to get their corners, criss cross the x's and y's:
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5corner1 = x1,y2
6corner2 = x2,y1
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8if the original points are on the same x-axis or same y-axis,
9the criss cross result will give you back the original points
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