Wow that sounds a lot more complicated than the one I had, HP 42S, because you had to use codes for the instructions instead of just pulling up the instructions in readable ASCII.
I found this random example of entering a program on a 42S: https://www.youtube.com/watch?v=JQMOOxup8bI
Coincidentally, it even uses the FP (fractional part) and X=0? test that the OP mentioned.
I wanted to see what programming the 15C looked like, so I found this as well: https://www.youtube.com/watch?v=m4dF5nF8I88 Looks like you can enter the program without knowing the codes, but reviewing the code just shows codes.
I found this random example of entering a program on a 42S: https://www.youtube.com/watch?v=JQMOOxup8bI Coincidentally, it even uses the FP (fractional part) and X=0? test that the OP mentioned.
I wanted to see what programming the 15C looked like, so I found this as well: https://www.youtube.com/watch?v=m4dF5nF8I88 Looks like you can enter the program without knowing the codes, but reviewing the code just shows codes.