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carved in stone
If a suggestion, plan, rule, etc. is carved in stone, it cannot be changed.
无法改变的;铁板钉钉的
These proposals are for discussion, they're not carved in stone.
这些提议是供讨论的，并非不可更改。
The deal isn't yet carved in stone, but we're confident it will go ahead as hoped.
这笔交易还不是板上钉钉的，但我们相信它会如预期进行下去。