I had an HP-48S in high-school (begged for an entire year as a birthday present—me a geek? Ha never), and I don’t think any of my teachers realised just how powerful that thing was, or how big its formula database was. Or that it could send data to other HP-48 users. 
I learned how to program it and made all kinds of useful functions. RPN forever! And thanks for the tip regarding HP emulators on the iPhone. I’ll have to check that out.