How can I limit the detection of squares to a limited field in the Mat frame?

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I would like to detect squares in a limited field of a detected frame, such as the first 200 pixels. I see the squares example, and have a general idea that I might only process squares that in the far left by specifying some maximum x-cordinate. I'm wondering if there are any functions I can make use of, to do this, or how I might set limits on the frame to be interpreted.

OpenCV, and computer vision newbie here