Earlier quoted context omitted.
If you're curious, the math-y term for an operation that is its own inverse is involution . I don't know that I've ever heard the word in a programming context, but there it is! :)
Thank you! A quick look through my bookshelf reveals that out of four books on discrete mathematics, only one contain involution in the index: and there it is mentioned once in a supplementary exercise... I guess that explains why I couldn't seem to recall a term for "self-inverse" [ed: No, that's wrong to, lol. Lets just stick with involution ].
The word "involution" finds more use in analysis and algebra, where self-inverses have more interesting (often geometric) properties. The only interesting property I can think of in the context of discrete mathematics would be that if S is a finite set and f:S → S is an involution then the parity of |S| is equal to the parity of the fixed points of f.
That is,
|S| ≡ |Fix(f)| (mod 2)