There are a lot of Mac OS X calculators that use RPN, and anyone who is involved much in math or science will be familiar with the term. The short explanation is that 3+5 becomes written 3 5 +, or 2*(3+8) becomes 2 3 8 + *, but that's leaving a lot out. You really do have to use it to understand it. All that said, while this is promising, it hogs too much memory and the pairing of Scheme, a prefix programming language, with RPN, seems bass-ackwards to me. Something like RPL (reverse-polish LISP, the language of HP calcs) seems like a better bet to me. As-is, if you know how to use the UNIX desk calculator, this program doesn't really offer much.