Beginner solution in Common Lisp!

I tried to go for a purely functional solution, but there were lots of things I didn't know how to do, so please feel free to help me make my solution prettier!

For example, since I haven't ever used lisp before, I had to translate the input string to a list, then the resultant list back to a string. I ended up using two separate functions for that, (a wrapper function an an "innards" function) but I'm sure there's a better way...