[plt-scheme] can't run SICP "generic operations" code in PLT Scheme 4.1

Hey all,

I can't figure out which language pack/ include statement / other
setting is necessary to run the generic operations package from SICP.

Here are two samples of the code I'm trying to run:

http://www.cs.uml.edu/~holly/91.301/ps6-code.ss
http://rlai.cs.ualberta.ca/325assignment5.html

The code works fine in PLT-Scheme 3.72 (setting "Textual (MzScheme,
includes R5RS)" but I am stumped trying to get it to run in 4.1.

In general 4.1 seems to not like redefining stuff... I must admit I am
mystified by all the language choices.

Thanks for your help,
Fred