sclass Fiber {
  Thread runningInThread;
  long grabCount;
  
  final Map<IVar, O> fiberLocalValues = syncWeakMap();

  synchronized void grab {
    Thread t = currentThread();
    if (runningInThread != null && runningInThread != t)
      failWithVars("Trying to assign virtual thread to 2 threads", +thread1 := runningInThread, thread2 := t);
    runningInThread = t;
    grabCount++;
  }
  
  synchronized void release {
    Thread t = currentThread();
    if (runningInThread != t)
      failWithVars("Broken release", +thread1 := runningInThread, thread2 := t);
    if (--grabCount == 0)
      runningInThread = null;
  }
  
  O getFiberLocalValue(IVar fiberLocal) {
    ret fiberLocalValues.get(fiberLocal);
  }
  
  void setFiberLocalValue(IVar fiberLocal, O value) {
    fiberLocalValues.put(fiberLocal, value);
  }
  
  void foretFiberLocalValue(IVar fiberLocal) {
    fiberLocalValues.remove(fiberLocal);
  }
}