sclass VirtualThread {
  volatile Thread realThread;
  
  Map<IVar, Map<Thread, O>> virtualThreadLocalData = weakMap();

  void setRealThread(Thread t) {
    if (realThread != null && realThread != t)
      failWithVars("Trying to assign virtual thread to 2 threads", +thread1 := realThread, thread2 := t);
    realThread = t;
  }
}