In how many ways can a dozen books be placed on four distinguishable shelves
if no two books are the same, and the positions of the books on the shelves matter?
(Hint: Break this into 12 tasks, placing each book separately. Start with the sequence 1,2,3,4 to
represent the shelves. Represent the books by bi, i = 1, 2, ..., 12. Place b1 to the right of one of
the terms in 1, 2, 3, 4. Then successively place b2, b3, ..., and b12.)