I’ve put up the source code for Impossible Stairs on IFArchive (where it now resides in the brand new /games/source/dialog directory). I don’t have very clean code in general, but I remember I struggled when writing the game because of the dearth of examples (mostly Craverly heights). I hope this is useful, and I definitely hope that others will post their own Dialog code to help us all out!
https://www.ifarchive.org/indexes/if-archive/games/source/dialog/