I tried this, and I'm not sure things played out as you said. After walking the Macro Stepper through "(cas-cad-e 2 ((1) 3))," I saw the unbound identifier error without really seeing two different colorings of mtc. But no big deal: I obviously don't understand hygiene as well as I thought I did, so I'll try to learn more about it on my own. Incidentally, I was surprised to get the same error when I made mtc a literal, along with go! and break. Does hygiene explain this too? <div class="gmail_quote">On Sat, Oct 9, 2010 at 2:58 PM, Shriram Krishnamurthi <<a href="mailto:sk@cs.brown.edu">sk@cs.brown.edu</a>> wrote: <blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">Unfortunately, each of those mtc's is different. Let me suggest you do this instead. Put the expression you typed at the > prompt in the Definitions window, and click on the Macro Stepper button. Step twice. The coloring is extremely significant (the only good use of syntax coloring I've seen in my life) -- it represents the hygienic macro expansion algorithm. If this coloring doesn't make sense to you, then you might want to write a much simpler macro just to explore how hygiene works. If this coloring does make sense to you, then you can go back to understanding its implication for your macro. Shriram </blockquote></div>