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>>2
It is a purely functional transformation of a data structure that allows easy and efficient navigation.
For example, a (cons) list zipper of the list (0 1 2 3 4) focused at position 3 could be represented as ((2 1 0) . (3 4)). As you can see, you can travel both forwards and backwards in the list in O(1), e.g. to travel backwards, you would remove the 2 in the first position of the first list and insert it as the first element of the second list.
You can also have tree zippers (which would have an 'up' navigation function instead of just 'left-child' and 'right-child'), as well as for pretty much any kind of data structure.
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