source: CIVL/examples/por/trade3.cvl

int x=0;

void p1() {
  $atomic {
    x=1;
    $yield();
    $when(x==2);
    x=3;
  }
}

void p2() {
  $when(x==1);
  x=2;
  x=4;
}

void main() {
  $proc proc1 = $spawn p1();
  $proc proc2 = $spawn p2();

  $wait(proc1);
  $wait(proc2);
  $assert((x==3)); // false
}