How to use the pygalmesh.Union function in pygalmesh

def test_cuboids_union():
    c0 = pygalmesh.Cuboid([0, 0, -0.5], [3, 3, 0.5])
    c1 = pygalmesh.Cuboid([1, 1, -2], [2, 2, 2])
    u = pygalmesh.Union([c0, c1])

    mesh = pygalmesh.generate_mesh(u, cell_size=0.2, edge_size=0.2, verbose=False)

    # filter the vertices that belong to cells
    verts = mesh.points[numpy.unique(mesh.cells["tetra"])]

    tol = 1.0e-2
    assert abs(max(verts[:, 0]) - 3.0) < tol
    assert abs(min(verts[:, 0]) - 0.0) < tol
    assert abs(max(verts[:, 1]) - 3.0) < tol
    assert abs(min(verts[:, 1]) - 0.0) < tol
    assert abs(max(verts[:, 2]) - 2.0) < tol
    assert abs(min(verts[:, 2]) + 2.0) < tol

    vol = sum(helpers.compute_volumes(mesh.points, mesh.cells["tetra"]))
    assert abs(vol - 12.0) < 0.1

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

    a = numpy.sqrt(radius ** 2 - displacement ** 2)
    edge_size = 0.1
    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, verbose=False
    )

    assert abs(max(mesh.points[:, 0]) - (radius + displacement)) < 0.02
    assert abs(min(mesh.points[:, 0]) + (radius + displacement)) < 0.02