void FixedUpdate()
{
var body = GetComponent <Rigidbody>();
body.velocity = new Vector3(Input.GetAxis("Horizontal") * speed, 0.0f, Input.GetAxis("Vertical") * speed);
body.position = boundary.Apply(body.position);
body.rotation = Quaternion.Euler(0.0f, 0.0f, body.velocity.x * -tilt);
}