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The tests that passed against code I deleted: four shapes of a test that cannot fail
I removed the one element on a dashboard that says where the money went, ran the suite, and all 867 of its tests passed. That is mutation testing: change the production code on purpose and see whether anything goes red. The survivors clustered into four recognisable shapes, each with a fix you can copy: a lone negative assertion with no positive twin, a denylist that does not cover its own space, a test satisfied by the library's own fallback, and a wire between two files that each have green tests. Every mutation here was applied by hand and re-run against two trees, before the fixes and after, with the numbers from today's run.
