module outset(r) {
    minkowski() {
        circle(r=r);
        children();
    }
}

module inverse() {
	difference() {
		square(1e5,center=true);
		children();
	}
}

module fillet(r=1) {
	inset(r=r) render() outset(r=r) children();
}

module rounding(r=1) {
	outset(r=r) inset(r=r) children();
}

module inset(r) {
    inverse() outset(r) inverse() children();
}


module shell(d,center=false) {
	if (center && d > 0) {
		difference() {
			outset(d=d/2) children();
			inset(d=d/2) children();
		}
	}
	if (!center && d > 0) {
		difference() {
			outset(d=d) children();
			children();
		}
	}
	if (!center && d < 0) {
		difference() {
			children();
			inset(d=-d) children();
		}
	}
	if (d == 0) children();
}