[vessenes]

This is just beautifully expressive python. I wouldn't think of using it this way: writing

  return (not know(possible_dates)
          and all(not know(tell(Day(d)))
                    for d in possible_dates))

feels almost zen to me; or at least like Mr. Norvig likes himself some lisp at times.

[logicallee]

Python: write pseudocode, and then make sure it's indented properly.

[selectiveshift]

He does in fact like himself some lisp: http://norvig.com/paip.html