“coffin” or killer problem - Oral entrance exams to the math department of Moscow State University

Construct (with ruler and compass) a square given one point from each side.

I was reading the answer to Square Deal and I do not understand how the rest is easy to solve. Could someone help me understand, I don't see the square.

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@Freddy, if it's not a Euclidean space, then what's a square? –  Gerry Myerson Jan 20 '13 at 10:15
Ah, Ok, I overlooked it. –  007resu Jan 20 '13 at 10:57
Connect $A$ and $C$ and draw a perpendicular from $B$ to this segment. Now construct $D’$ on this perpendicular such that $AC = BD’$. $DD'$ is one side of the square. (This construction is already given in the link provided)
Now to complete the square, draw perpendiculars from point $A$ and $C$ to the line $DD'$. And then draw a line passing though $B$ parallel to $DD'$.