THEM: i'd like 2 books and 1 hat. you can have the balls.  YOU: that will not work. are the balls very important to you?  THEM: they're all important, but if you need the hat, i can take the balls in exchange.  YOU: ok, may i take the books and give you everything else?  THEM: sorry, that doesn't work. i need the books, and at least 1 hat. or the books and the 2 balls.  YOU: give me one book and take everything else. final offer.  THEM: deal!