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The definitions of sml:key and sml:unique state that the semantics are essentially the same as but can also be used . I think you mean that the semantics are exactly the same except for the one difference noted. The phrase essentially the same is fuzzy. To me it suggests that there may be other differences. Note that this same language was used in keyref in the LC draft but has been changed in the current editor's draft. A similar change seems appropriate for key and unique.
As per resolution in 5/22 call, this is purely editorial
Changed "The semantics are essentially the same as that for ... but ..." to "The semantics are the same as that for ... except that ..."