int lock0 = 0;
int lock1 = 0;

void proc0() {
  $when (lock0==0) lock0=1;
  $when (lock1==0) lock1=1;
  lock0=0;
  lock1=0;
}

void proc1() {
  $when (lock1==0) lock1=1;
  $when (lock0==0) lock0=1;
  lock0=0;
  lock1=0;
}

void main() {
  $spawn proc0();
  $spawn proc1();
}