RPL, the threaded interpreted language used on HP's Saturn based calculators since the HP-18C, was developed by William C. Wickes back around 1983 to 1984. Some of the information in this article is available from the journal article "RPL: A Mathematical Control Language" by William C. Wickes, published in "Programming Environments", Institute for Applied Forth Research, Inc., 1988.
Hello I'm the author of the referenced article. It's currently only in draft form and I've been updating and editing it to make it clearer and more understandable. If there's any ambiguity, mistakes or anything that causes confusion or isn't clear enough, please let me know
I next plan on adding a bunch of diagrams to the article which should improve its understandability
Oh, welcome! And thanks for writing and publishing your document.
Diagrams certainly help me!
You're certainly welcome!
I'll probably add ASCII art diagrams first as I'm unfamiliar with Inkscape which I'm using to draw the graphical diagrams and I'm pulling my hair out over things not going right in Inkscape
Good plan - I like a bit of ASCII art. We had a thread collecting suitable tools: viewtopic.php?f=1&t=3109
Thanks for the link
A day or so ago I performed a major rework / edit on the article to fix style issues and to make it clearer and more understandable guided by some suggestions from other HP Museum forum members ( I also added some more information ). I'll next be adding ASCII "graphics" first as I'm using Inkscape for the images but I'm in the process of learning how to use Inkscape.
I'm the author of the referenced article. It's currently only in draft form and I've been updating and editing it to make it clearer and more understandable. If there's any ambiguity, mistakes or anything that causes confusion or isn't clear enough, please let me know 
first as I'm using Inkscape for the images but I'm in the process of learning how to use Inkscape.