Back in 1961, John McCarthy (the inventor of LISP) described an interesting mathematical operator called amb. Essentially, amb hates to be called with no arguments, and can look into the future to keep that from happening. Here’s how it might look in Ruby. # amb will (appear to) choose values # for x and y that prevent future # trouble. x = amb 1, 2, 3 y = amb 4, 5, 6 # Ooops! If x*y isn't 8, amb would # get angry. You wouldn't like # amb when it's angry. amb if x*y != 8 # Sure enough, x is 2...