refined notion of empty toplevel, admits undo of 'end';
added undo_exit, init_empty, init_state;
removed unused toplevel, reset;
tuned;
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