function updateFragment(
    returnFiber: Fiber,
    current: Fiber | null,
    fragment: Iterable<*>,
    expirationTime: ExpirationTime,
    key: null | string,
  ): Fiber {
    if (current === null || current.tag !== Fragment) {
      
      const created = createFiberFromFragment(
        fragment,
        returnFiber.mode,
        expirationTime,
        key,
      );
      created.return = returnFiber;
      return created;
    } else {
      
      const existing = useFiber(current, fragment, expirationTime);
      existing.return = returnFiber;
      return existing;
    }
  }
  function createChild(
    returnFiber: Fiber,
    newChild: any,
    expirationTime: ExpirationTime,
  ): Fiber | null {
    if (typeof newChild === 'string' || typeof newChild === 'number') {