That was a great watch...and if you keep on watching for a few more minutes, you can watch Lorinda L. Cherry rewrite his implementation using Unix pipes and then creating a talking calculator. The screencasting technology wasn't quite there yet ;), but both are really excellent demonstrations and eludications of the Unix philosophy and mindset that is pretty much unchanged and invaluable ~34 years later https://youtu.be/XvDZLjaCJuw?t=13m47s