Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2003/functional-programming-data-structures-and-algorithms-procedural-programming-and-digital-hardware/2/a/iii/solution

Idempotence concerns applying an operation to the same whole value. It says more than saying that the equal-entry pairs are unchanged. Choose the idempotent binary operation min. Then
More generally, a finite list of length paired with itself produces entries, so length alone contradicts idempotence when . Therefore

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