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I don’t know about this particular one, but many of these projects make use of Thomas Okken’s amazing Free42[1] which is a GPL-licensed emulator of a superset of the HP42S, which lots of RPN nerds consider the apex of that type of calculator. By superset I mean HP42S programs run entirely unaltered but it has 4 or 5 keywords added which are entirely in the spirit of the original and (in my opinion) feel like they should always have been there. For example the HP42S has excellent support for complex numbers. The CPLX function takes the contents of the Y and X registers and turns them into a complex number. If you’re in rectangular mode then they are the i and j components of the number in cartesian form and if you’re in polar mode they are r and theta. However that means if you’re writing a function it’s a pain in the ass to make it work in both cartesian and polar mode. Free42 adds two new keywords PCPX (which means “like you hit CPLX but it’s always in polar mode”) and RCPX (the same but for rectangular). As someone who predominantly uses rectangular it’s great to have PCPX on my custom menu for quick access.

In a similar vein it has a mode (which is actually what I use on my DM42 almost[3] all the time) where you have stack depth limited only by memory.

Aside from the app which is free, if you want a great physical calculator based on the free42 I would recommend the SwissMicros DM42[2] which I have and love. The only thing that I like better on my actual HP calculators is the keys, which the swissmicros is a bit too stiff especially since I have joint problems in my fingers.

[1] https://thomasokken.com/free42/

[2] https://www.swissmicros.com/product/dm42

[3] Except if I'm doing stats. Some of the built-in statistical functions don't work in n-stack mode