Write Barrier: was: [Rd] function-like macros undefined

* suppose that inside a C function I have a SEXP vector x of integers
and I want to increment each element by one. I understand that

Please correct me if I'm wrong, but I thought that the write barrier 
applies to assignments of SEXP values only (from the doc: "... must be 
used for all assignments of SEXP pointers ..." - note it says "of", not 
"to"). When dealing with REAL/INTEGER, I believe it's still safe to use 
INTEGER(x)[0]=... IMHO that's logical, too, because primitive types 
have no 'age' to be inherited when using ggc.

Cheers,
Simon