[plt-scheme] indenting algorithms for Scheme code?

I'd like to have a Java window (outside of PLT-Scheme) that can indent Scheme expressions as they are entered.  I've looked at how Emacs does it, but it winds up calling a built in function that keeps track of a 15 element state vector.  I keep thinking its like reading what you've typed backwards until you find a landmark like a "(let ".   Is there a programming pearl hear i should know about?

k