group() {
	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);
	}
}
