returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    textContent: string,
    expirationTime: ExpirationTime,
  ): Fiber {
    // 
    // 
    if (currentFirstChild !== null && currentFirstChild.tag === HostText) {
      // 
      // 
      deleteRemainingChildren(returnFiber, currentFirstChild.sibling);
      const existing = useFiber(currentFirstChild, textContent, expirationTime);
      existing.return = returnFiber;
      return existing;
    }
    // 
    // 
    deleteRemainingChildren(returnFiber, currentFirstChild);
    const created = createFiberFromText(
      textContent,
      returnFiber.mode,
      expirationTime,
    );
    created.return = returnFiber;
    return created;
  }
  function reconcileSingleElement(