public prefix func ++ (vector: inout Vector3D) -> Vector3D {
    vector += Vector3D(x: 1.0, y: 1.0, z: 1.0)

    return vector
}

var toIncrement = Vector3D(x: 1.0, y: 2.0, z: 3.0)
let afterIncrement = ++toIncrement

/* Since the operator parameter is inout, toIncrement now has: x = 2, y = 3, z = 4 */
// afterIncrement also has: x = 2, y = 3, z = 4 */