minkowski(convexity = 0) {
	difference() {
		square(size = [40, 40], center = true);
		minkowski(convexity = 0) {
			circle($fn = 64, $fa = 12, $fs = 2, r = 2);
			circle($fn = 64, $fa = 12, $fs = 2, r = 4);
		}
	}
	square(size = [1, 1], center = false);
}
