Write tests for code that already works

Write tests for the code below. It currently works; I want to keep it that way.

<code>
{{CODE}}
</code>

Rules:
- Test the behaviour, not the implementation. If I refactor the inside, the tests should pass.
- One test per behaviour, named so a failure tells me what broke without opening the file.
- Cover the edge cases that look deliberate in this code — empty input, the boundary values,
  the error path.
- Do not test getters, or that a constructor assigns its arguments.
- If you cannot tell what some behaviour is for, list it as a question instead of guessing.