[plt-scheme] htdp section 17.7

http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-22.html

re 17.7.2 - Am I right to say that the data definitions for parameter name 
and expression have to allow for "nulls". I ask with particular reference to 
problems 3 and 5 which seem to require a null parameter and a null 
expression respectively.

re 17.7.3 - Can I have an example of what is required as I don't 
sufficiently "get" the problem as it is set.

thanks.