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As I had casually mentioned at this article, the HP-12C set of features is a bit dated. In my opinion,
Given these considerations, I put together the 12C of my dreams:
Besides the proper Black-Scholes model (with the classical five 'greeks') and implicit volatility, it has ΔDYS252 to count business days, taking full account of moving holidays.
I would have liked to remove more original features, but I felt that remaining items are all useful and necessary. In the other hand, the original 12C has a "surplus" of function points — keys with no assigned function for modifiers "f" and "g" — so I had room to add everything I wanted.
Here's a video about the new features: