returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChildrenIterable: Iterable<*>,
    expirationTime: ExpirationTime,
  ): Fiber | null {
    // 
    // 
    const iteratorFn = getIteratorFn(newChildrenIterable);
    invariant(
      typeof iteratorFn === 'function',
      'An object is not an iterable. This error is likely caused by a bug in ' +