funzione tick

def tick():
    arena.move_all()
    if g2d.key_pressed("ArrowUp"):
        rover.go_up(True)
    elif g2d.key_released("ArrowUp"):
        rover.go_up(False)
    if g2d.key_pressed("ArrowRight"):
        rover.go_right(True)
    elif g2d.key_released("ArrowRight"):
        rover.go_right(False)
    if g2d.key_pressed("ArrowDown"):
        rover.go_down(True)
    elif g2d.key_released("ArrowDown"):
        rover.go_down(False)
    if g2d.key_pressed("ArrowLeft"):
        rover.go_left(True)
    elif g2d.key_released("ArrowLeft"):
        rover.go_left(False)

      # Game logic

    g2d.clear_canvas()
    for b in backgrounds:
            b.move()
            g2d.draw_image_clip(backgroundsimage, b.symbol(), b.position())

    for a in arena.actors():
        if a.symbol() != (0, 0, 0, 0):
            g2d.draw_image_clip(sprites, a.symbol(), a.position())
        else:
            g2d.fill_rect(a.position())