CSC151 2009F, Class 27: Recursion Basics, Continued

Admin:
* I've rearranged the schedule a bit to better accommodate last Friday's 
  pause for breath.
* Reading for Wednesday: Recursion with Helper Procedures.
* You'll find it useful to complete today's lab (if I ever give you time
  to start it)
* Are there further questions on Assignment 6?
  * For part three, can we use image-select..
    I'd prefer not
* Recursion can be puzzling, but I think you'll puzzle it out better
  by starting the lab and asking lots of questions.

Overview:
* Lab!

Recursion:
* Basic idea: 
  * Classify the input
    * Simple enough to solve immediately, do so
    * Not so simple
      * Call the *same* procedure on a simpliefied version of the input
        (with lists, almost always the cdr of the list)
      * Combine the result of that call with other stuff to compute a result