* Writing and reading tables to external files
(Preferences file loaded on replaying; Alien Invasion Part 23)
Keeping a preference file that could be loaded by any game in a series.

Suppose that we have a series of games each of which allows the player to select a puzzle difficulty level. When the player plays a new game in the series, we want him to start out by default with the same difficulty level he faced earlier on, so we store this information in a small preferences file, as follows:

	{*}"Alien Invasion Part 23"

	A difficulty is a kind of value. The difficulties are easy, moderate, hard, and fiendish.

	Understand "use [difficulty] puzzles" as selecting difficulty. Selecting difficulty is an action out of world, applying to one difficulty.

	Carry out selecting difficulty:
		choose row 1 in the Table of Preference Settings;
		now challenge level entry is difficulty understood;
		say "Puzzles will be [challenge level entry] from now on."

	The File of Preferences is called "prefs".

	When play begins:
		if File of Preferences exists:
			read File of Preferences into the Table of Preference Settings;
			choose row 1 in the Table of Preference Settings;
			say "(The current puzzle difficulty is set to [challenge level entry].)"

	Check quitting the game:
		write File of Preferences from the Table of Preference Settings.

	Table of Preference Settings
	challenge level
	easy

	The Sewer Junction is a room.

Our preference file is restricted to a single option here for simplicity's sake, but we could keep track of more information -- whether the player preferred verbose or brief room descriptions, screen configurations, and so on.
	
If we were disposed to be somewhat crueler, we could use a similar method to make the player finish each episode of the series in order to "unlock" the next. All we would need to do is store a numerical password in our preferences file when the player finished a given level; the next level would check, say, the Table of Completed Levels for that password, and refuse to play unless the right number were present.