How to use the pygalmesh.Difference function in pygalmesh

def test_balls_difference():
    radius = 1.0
    displacement = 0.5
    s0 = pygalmesh.Ball([displacement, 0, 0], radius)
    s1 = pygalmesh.Ball([-displacement, 0, 0], radius)
    u = pygalmesh.Difference(s0, s1)

    a = numpy.sqrt(radius ** 2 - displacement ** 2)
    edge_size = 0.15
    n = int(2 * numpy.pi * a / edge_size)
    circ = [
        [0.0, a * numpy.cos(i * 2 * numpy.pi / n), a * numpy.sin(i * 2 * numpy.pi / n)]
        for i in range(n)
    ]
    circ.append(circ[0])

    mesh = pygalmesh.generate_mesh(
        u,
        feature_edges=[circ],
        cell_size=0.15,
        edge_size=edge_size,
        facet_angle=25,