[plt-scheme] Explaining assignment

Matthias wrote:

> I routinely teach these parts (when I teach them) with just those  
> explanations. I believe the communication idea is somewhere in this  
> chapter but I will admit that I was afraid of complaints about the  
> nature of my English explanations. So I may have tried to water it down.

I think the early use of the address book example in HtDP makes this 
idea of communicating between functions (or between different 
applications of the same function) quite clear.

There are further puzzles with how Advanced Student handles the void 
value, though. Sometimes (void) is printed in the Interactions window 
and sometimes it is not. I have not been able to figure out the rules 
for this.

In our course for majors, we skip over Advanced Student, and move into 
Module, covering interaction (display/printf/read-char/read) before 
mutation. There I make the role of #<void> clear. But our course for 
non-majors uses Advanced Student, and I think the anomalous role of 
(void) is a bit confusing to students. --PR