Set operations such as union/intersection.

This file defines a generic tree walker for Clojure data
structures.  It takes any data structure (list, vector, map, set,
seq), calls a function on every element, and uses the return value
of the function in place of the original.  This makes it fairly
easy to write recursive search-and-replace functions, as shown in
the examples.

Note: "walk" supports all Clojure data structures EXCEPT maps
created with sorted-map-by.  There is no (obvious) way to retrieve
the sorting function.