This is one of the times you feel stupid. You think you know quite well what you have, and there comes someone that shows you know nothing.
Today, at work, I was using my HP50g, and a companion said to me: "wow, that's the calculator that comes with Tetris".
"You mean you can load games, do you? of course you can find a lot of games for it", I replied.
"No, I mean that it comes with Tetris in its software", he said.
"I don't think so".
"Try this: open the formula editor (right arrow + EQW), set alpha, and write MINEISBETTER".
"Now, select it:"
"and now, de-select alpha and press "SIMP" (F6). Voilà!!"
You play it with the keys 2, 4 and 6 for movement across the screen, and 5 to rotate. Backspace to stop it.
Again: all the time in front of my nose, and I did not know!!