Various cheat codes

These codes are debugging tools that the programmers left in. They are:
Code ....... Effect
TAB + 2 + L Extra Life
TAB + 2 + F Full Fuel Refill
TAB + 2 + I Invincibility
TAB + 2 + S Skip a Level

Secrets
Fast snow

In the Christmas Edition, hold down S at the title screen to make the snow fall really fast and build up quickly.