Parser being too smart with fold expressions

/tmp|=> cat test-fold.sac 

int main () {
  a = [1,2,3];
  return with {
    ([0] <= iv <= _shape_A_ (a)): _sel_VxA_ (iv, a);
  }: fold (_add_SxS_, 0);
}

/tmp|=> ./sac2c/build_d/sac2c_d test-fold.sac

**  1: Loading SAC program ...
  **** Locating source code ...
  Reading from file "./test-fold.sac" ...
  **** Running C preprocessor ...
  **** Parsing input file ...
./test-fold.sac:5:12: error: 
=> invalid function name `_add_SxS_' found
./test-fold.sac:7:1: error: 
=> token `}' expected, `EOF' token found
abort: Failed to construct a syntax tree for `test-fold.sac'
compilation failed while Loading SAC program, 2 error(s).

The problem is that parser expects an identifier, but _add_SxS_ is a keyword, hence the error. This can be trivially fixed, given that the backend is happy to accept primitive function as a valid object. The question is: what should SPFOLD_FN be in this case? SPID or PRF?