must .Call C functions return SEXP?

Because R assumes that you don't mess with the arguments, it also
optimizes b to point to the same object as a which you then modify.

I hope it sheds some light on it.

Indeed!  Thank you Simon, that was a thorough and illuminating answer.

(Now I have better understanding of how the machine works, and what
the dangerous sharp bits are.  :)

Andrew Piskorski <atp at piskorski.com>
http://www.piskorski.com/