Imperative programming in scheme

So I know I can redefine variables with (set! var), but how would I do other imperative things, like for loops?

>>3
(loop ...)

Thanks.
>>4
I'm trying to annoy the living fuck out of my professor. He was going off about how you can't redifine constants in Scheme, that technically there was one way to do it, but he wouldn't show us till the end of the quarter.... so I looked up how to do it. Now I just want to see how I could make the ugliest imperative beast of scheme possible.

In all fairness, I do value learning functional programming... even if I don't see myself using it much in the future.