Almost every programmer has faced the problem of
representing a tree together with a
subtree that is the focus of attention, where that focus may
move left, right, up or down the tree. The Zipper is Huet's nifty name for a nifty data structure which fulfills this need. I wish I had known of it when I faced this task, because the solution
I came up with was not quite so efficient or elegant as the Zipper.