Bug in class implementation

Here is a definition of the class which is a counter, where the update effect is merged into the value accessor:

class cnt;
classtype int;
export all;

cnt new (int x) {
  return to_cnt (x);
}

int value (cnt &x) {
  v = from_cnt (x);
  x = to_cnt (_add_SxS_ (v, 1));
  return v;
}

Here is the file that uses this class:

use cnt:all;
int main () {
  a = new (1);
  return value (a);
}

When I run this, I get the value 2 back, but the correct result is 1.

The version of sac2c I used is: sac2c 1.3.3-MijasCosta-1161-gb543c