docs: CancellationTokenRegistration register(Runnable action) {
      CancellationTokenRegistration ctr;
      synchronized (lock) {
        throwIfClosed();
  
        ctr = new CancellationTokenRegistration(this, action);
        if (cancellationRequested) {
          ctr.runAction();
        } else {
          registrations.add(ctr);
        }
      }
      return ctr;
    }