Directed

canvas.duration(0).zoom(0.7)

canvas.node(1).add()
for (let n = 1; n < 20; n++) {
    const [n1, n2] = [n * 2, n * 2 + 1]
    canvas.nodes([n1, n2]).add()
    canvas.edges([[n1, n], [n2, n]]).add({ directed: true })
}