function draw_apollonian_circle(svg, xa, xb, q) {
const [x1, x2] = get_diameter(xa, xb, q);
const x0 = (x1 + x2) / 2;
const r = Math.abs(x2 - x1) / 2;
const da = Math.abs(x1 - xa);
const db = Math.abs(x1 - xb);
draw_circle(svg, x0, r)
.attr("stroke", "blue")
.attr("fill", "none");
}