set in stone 板上钉钉，表示绝对不变。
not be set/carved in stone = to be able to be changed 还未确定；未成定局。

【例句】
These are just a few ideas - nothing is set in stone yet.
这还只是一些想法——什么都还没定下来。

Tyler：Well, my plans aren't set in stone, so I think I can help you then.
泰勒：喔，我的计划还没有完全确定，所以我想我到时候可以帮妳。