it works in pretty big ,lazy scheme and also FRtime plus it works in PLAI scheme. My drscheme version : 4.1.4 <div class="gmail_quote">On Sat, Apr 4, 2009 at 11:12 PM, dave yrueta <<a href="mailto:dyrueta@gmail.com">dyrueta@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Hi All -- After creating function 'create-tests,' I ran the following per HtDP exercise 26.3.6 -- (define test-case (create-tests 10000)) (collect-garbage) (time (sort test-case)) (collect-garbage) (time (quick-sort test-case)) -- which returned this: collect-garbage: name is not defined, not a parameter, and not a primitive name I tried changing the teaching language to Advanced Student and still received the same error message. Is there something I need to do to enable the collect-garbage function? Also, what does collect-garbage do? Thanks! Dave Yrueta _________________________________________________ For list-related administrative tasks: <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </blockquote></div>