THEM: hmm. i want the book and 3 of the balls.  YOU: i want the book and the hat and you can have all of the balls.  THEM: i need the book. you can have the hat and all the balls.  YOU: i have to have the book. let's negotiate on the other stuff.  THEM: i can't accept any deal that doesn't include the book in my package.  YOU: alright, alright - you can have the book. i will take all the rest.  THEM: agreed.