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What is different is the incremental iterative approach to building out the landscape. Also, in the structured model, you would attempt to define all the input and output values before any coding could be done. Not so with TDD. Instead, everything is done incrementally and iteratively. Even the identification of the tests.
Is it possible to perform unit testing with tools like
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FitNesse?
Yes it is but why would you? You would actually be using the wrong tool for the particular job. It would be like using a hammer to put a screw in the wall. Yes it would work and the results would be the same, but the approach at that level of development would be cumbersome.