Forth Calculator (RPN) final version - Processing 2.x and 3.x Forum
File:Vintage Hewlett-Packard Model HP-16C Progammable RPN Calculator, HP's First and Only Programmable Calculator, Circa 1988 (9330369298).jpg - Wikimedia Commons
CAM #4 - Pt 2 What is RPN Reverse Polish Notation / How to use RPN on the HP 12C, HP 15C and HP 50g - YouTube