Test-Driven Development

Core Rule

Work in vertical red-green-refactor slices:

1. RED: write one test for one observable behavior.
2. GREEN: implement the smallest production change that makes that test pass.
3. REFACTOR: clean up only while tests are green.

Do not write a batch of imagined tests before implementation. Each new test should respond to what the last cycle taught you about the actual behavior and interface.

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