RPN Calculator

This calculator is based on the algebraic calculator tutorial presented in Simson Garfinkel & Michael Mahoney's "Building Cocoa Applications: A Step-by-Step Guide"
( http://www.oreilly.com /catalog/buildcocoa/ ), which I highly recommend. To it I have added the ability to enter fractional numbers, as well as the aforementioned RPN operation. The stack is unlimited. You can enter numbers until you run out of memory before you start doing operations if you like. In a future version, I'd like to add a paper tape like in the new OS X calculator, and possibly proper backspacing like an HP calculator does, instead of the clear entry function it currently has.