• Make sure that the state info is displayed correctly on the top-right corner.
• When in state |+> or |->, the user can move only the enemies up and down.
• When pressing arrow keys "UP" or "DOWN", move every enemy accordingly.
• Enemies can cross the middle line, and when dropping from the top (resp., bottom), they appear again from the bottom (resp., top).
• Enemies should still move automatically to the left.