[plt-scheme] A tutorial on simple zippers

Hi everyone,

I think I finally understand the zipper data structure.  I cooked up a 
tutorial trying to explain them to myself, and thought it might be helpful 
to others:

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

I try to stick with HTDP terminology as best as I can, but any suggestions 
to simplify would be greatly appreciated.  Thanks!