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A unicursal curve in the plane is a curve that you get when you put down your pencil, and draw until you get back to the starting point. As you draw, your pencil mark can intersect itself, but you're not supposed to have any triple intersections. You could say that your pencil is allowed to pass over an point of the plane at most twice. This property of not having any triple intersections is generic: if you scribble the curve with your eyes closed (and somehow magically manage to make the curve finish exactly where it began), the curve won't have any triple intersections.
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