I mean "sure" only as a manner of speaking, not to say that I am absolutely certain. The challenge for me in using the illegal opcodes is internalizing them and recognizing the situations where they can actually be used effectively to save a byte or a cycle. When writing code for platforms with very little memory or harsh time constraints, they can certainly save you headaches. Something like LAX for example can be used to do an y-indexed load to the x register, which often times can save you an accumulator transfer when doing doubly indirect loads.
Sorry for the inconvenience. I am not familiar with the details of English language, so sometimes I just understand things wrong.
Yes, there are some interesting opcodes too, but I found their usage so limited, that it was not worthwhile to drop compatibility. Of course, with a fixed setting, where you need every byte .... but I must say, that for me today it is just academic, since I also do no embedded programming today. With computers of today, I still think, that optimizing is worthwhile, but not to that degree, that breaking compatibility makes sense.
