identify: more informative id in Toplevel.debug mode;
interactive state transformations: dispose descendants *before* running next command, bypass for control commands;
editor model: actually run affected commands;
misc tuning;
By chapter:
chapters 1 and 2, maybe 3: Peter White
chapters 6, 7 and 8: David von Oheim
chapter 6, maybe 8: Gerwin Klein
chapters 3, maybe 4: Tanja Vos
chapters 1-3 at least: Stefano Bistarelli