returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChild: any,
    expirationTime: ExpirationTime,
  ): Fiber | null {
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    if (
      typeof newChild === 'object' &&
      newChild !== null &&
      newChild.type === REACT_FRAGMENT_TYPE &&
      newChild.key === null
    ) {
      newChild = newChild.props.children;
    }
    // 
    const isObject = typeof newChild === 'object' && newChild !== null;
    if (isObject) {
      switch (newChild.$$typeof) {
        case REACT_ELEMENT_TYPE: