[racket] doubly linked list lib

> Nothing obvious, no. But Racket is designed to encourage programming without
> mutation, and doubly linked lists require mutation.
>
> It's very likely a zipper will do what you want. A zipper is much easier to
> implement than a doubly linked list, and has similar performance and uses.


To supplement, here are very old notes I wrote to myself on the kind
of list zipper that Neil is presenting:

    https://hkn.eecs.berkeley.edu/~dyoo/plt/zippers.txt