This is my programmer's calculator, but two undocumented functions has been added.
1) Named secret(), takes two arguments. What does it do? Hint: running it with various arguments can give a hint.
https://github.com/DennisYurichev/progcalc (Jul 27 2017) Modified version for https://challenges.re/88/  secret(1,2)  (unsigned) 4 0x4 0b100  secret(1,6)  (unsigned) 8 0x8 0b1000  secret(2,4)  (unsigned) 11 0xb 0b1011  secret(3,3)  (unsigned) 61 0x3d 0b111101 ASCII: '='
It is a well-known function, the article about in in Wikipedia has ~300 in links (what links here).
2) No name known. Try to find it.
Downloads: Win64, Linux64.
More challenges: challenges.re; about solutions: challenges.re/#Solutions.