§1.7. The Skein
The Replay button demonstrates that Inform must be quietly remembering the commands typed into the last run through the game. In fact it remembers, and automatically organises, every previous run.
Inform's approach to testing interactive fiction is to treat it as being like the analysis of other turn-based games, such as chess. It would be prohibitively difficult to work out every possible combination of moves: instead, we analyse those which go somewhere, and look for significant choices. Every Queen's Gambit begins with the same first three moves (1. d4, d5; 2. c4), but then there is a choice, as the next move decides whether we have a Queen's Gambit Accepted (dxc4) or Declined (e6). Books about chess often contain great tables of such openings, which run together for a while but eventually diverge. To learn chess, one must explore all of these variations.
Inform's Skein panel is just such a table, built automatically. If we think of the list of typed commands as a thread, then the skein is (as the name suggests) braided together from all these threads. In the display, time begins at the top, with the start knot, and the threads of different games hang downwards from it.
Double-clicking on a command translates the source afresh and replays the game from start down to that command, and then stops. We are then free to continue play by typing commands into the Game panel, of course, and these commands will automatically be recorded in the Skein as a new variation of play, diverging from the previous threads.