public func += (left: inout Vector3D, right: Vector3D) {
    left = left + right
}

var initialVector = Vector3D(x: 1.0, y: 2.0, z:3.0)
let vectorToAdd = Vector3D(x: 3.0, y: 4.0, z:5.0)

initialVector += vectorToAdd /* initial vector now has: x = 4, y = 6, z = 8 */