example to learn pygame library. 2d game
##DESCRIPTION
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In Alien Invasion, the player controls a rocket ship that appears at the bottom center of the screen.
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The player can move the ship right and left using the arrow keys and shoot bullets using the spacebar.
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When the game begins, a fleet of aliens fills the sky and moves across and down the screen.
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The player shoots and destroys the aliens. If the player shoots all the aliens, a new fleet appears that moves faster than the previous fleet.
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If any alien hits the player’s ship or reaches the bottom of the screen, the player loses a ship.
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If the player loses three ships, the game ends.