Press [F3] + [F4] to set the internal level counter to the next level, then press [Esc] to display the pause menu and select "Continue" to advance to that level. Press [F1] + [F2], then press [Esc] and select "Continue" to go to the previous level.
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Hide from enemies
When low on health, hold 9 to increase the fog. Your enemies will not be able to see you, and you will be save unless they are very near to you. To remove the fog, hold 0.