Pause the game and type in "RUSSIAN AIR".  Unpause, and now pressing
  <N> will skip a stage.  Note: I don't think this works!
  What And Where To Buy
  ---------------------
  Level 1, Shop 1: Buy Health, Super Nashwan
  Level 1, Shop 2: Sell Rear Shot, Buy Double Shot and Side Shot
  Level 2, Shop 1: Buy Side Shot
  Level 2, Shop 2: Sell Side Shot
  Level 3, Shop 1: Buy Health and Rear Shot.
  Level 3, Shop 2: Sell Rear Shot, Buy Side Shot, Laser, and Power Up
  Level 4, Shop 1: Sell Rear Shot, Sell Side Shot
  Try to stick with your best weapon, and end up with 2 Cannons, 2 Lasers,
  and either Side Shot or Rear Shot.  Always spend your money as you cannot
  take leftover cash.
  Halfway and End-Of-Level Monsters
  ---------------------------------
  Snail:    Shoot at eye, avoid mines.
  Alien:    Shoot eyes at either side at top.  Move back and shoot centre
            eye.
  Spider:   Destroy as much web as possible.  Fly down to face spider and
            fire continuously.
  Crayfish: Travels in figure eight.  Stay below and fire in it's face.
  Snake:    Attacks from left of screen.
  Dragon:   Shoot the side heads first, then tail, then main head.
  Head:     Shoot both eyes, avoiding laser fire and extending tongue.  Shoot
            at head when tongue goes in.
  Tank:     Use missiles.
  Gand Spaceship: Shoot all cannons until totally destroyed.
          ITEM             COST         COMMENTS
  +--------------------+---------+--------------------------------+
  | Advice             |    200  | Only useful after Level 1      |
  | Speedup            |    500  | Essential to survival          |
  | Half Health        |    500  | Restores half your energy      |
  | Autofire           |    500  | Needed if joystick has none    |
  | Nashwan            |    600  | 10 Seconds of sheer heaven     |
  | Full Health        |   1000  | Restores full energy           |
  | Rear Shot          |   1000  | Very effective on later levels |
  | Small Mine         |   1000  | Useless                        |
  | Side Shot          |   1000  | Cannot be used with Rear Shot  |
  | Electroball        |   1200  | Too sensitive to control       |
  | Power-Up           |   2000  | Double bullet size             |
  | Large Mine         |   3000  | Useless                        |
  | Double Shot        |   3000  | Recommended on later levels    |
  | Cannon             |   4000  | Effective when using two       |
  | Dive               |   4000  | Waste of Money, but fun        |
  | Missiles           |   4000  | Weak to start with             |
  | Laser              |   4000  | Awesome when using two or more |
  | Drone              |   4500  | Two slow                       |
  | Flamer             |   5000  | Extremely limited range        |
  | Bomb               |   5500  | Limited range but powerful     |
  | Extra Life         |   6000  | You'll need lots of these!     |
  | Homers             |   6000  | Four slow but powerful missiles|
  | Protection         |   6000  | Doesn't do much                |
  | Bitmap Shades      |   6000  | Darkens Screen - useless       |
  +--------------------+---------+--------------------------------+