15
(defthm f-is-monotonic
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(=< (f X) (set-union X (S)))))
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; applyf is constrained, so we don't care about its guards.
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(declare (xargs :measure (nfix n)))
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(applyf (f X) (1- n)))))
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(applyf nil (cardinality (S))))
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(applyf (S) (cardinality (S))))