[racket] HtDP, andmap & ormap

   Yes, an assertion would have been fine. You're right. 

Thanks, Matthias, and I apologize again.  I look forward to reading
your revised Part IV.  I learned a lot from reading current versions
of HtDP.  And if I may say, I like using ormap in this way:

You can search a Sudoku puzzle for a Visible Pair (2 cells in some
unit having the same 2 possibilities) that's useful by
(define (findVP Sudoku) (ormap proc listofPairs))
using the list of all cell-pairs in all the units, where `proc' is
defined using `and' to return either false or the Visible Pair plus
the list of cells with possibilities eliminated by the Visible Pair.