[wruza]

Iow, we flip some bits in `a`, then do subj, then mask it back to `a`. It’s unclear what it computes in general, but if `b` disturbs the 2-powerness of `a` then I guess we learn that fact by seeing zero. Not sure where to use it.

[rokicki]

There's a nice elegant description of what it does, mathematically, and a significant use in Computer Science.