Yeah, now here's the really bad news with this approach. Do any of your prospective 'customers' know anything at all about the TI-89, or basic (BASIC) programming? If so, and if I recall correctly, it's interpreted, and not compiled (as might be the case with an Assembly program). That means that anybody that opens up the program can see where you did this comparison. Unless you put in random crap, nonsensical loops and made gratuitous use of arbitrary jump tags. Or you could do it in assembly, which would take a more determined effort, which means that unless you have some really blinding insight (or unique programming ability--ever watch Blackhawk Down
? "Typing is a specialist skill?", "Can you
type?") then somebody else will write their own.
...But really, your program is what? A few lines of code that probably took an hour or two to write? You'd be better off just not sharing the program (retaining the advantage for yourself) or sharing it with the whole class (being the putative 'man of the hour' for comparatively little effort--may want to target those of your preferred gender), rather than putting in this marginal 'copy protection'. Maybe you can get the best of both worlds and say, "Used my program? Buy me / [Let me buy you] a beer!"
I won't even charge you my $4.75/copy royalty fee for the copy protection scheme!