Rules of TDD:

• Write production code only to pass a failing unit test.
• Write no more of a unit test than sufficient to fail (compilation failures are failures).
• Write no more production code than necessary to pass the one failing unit test.

Transformation Priority Premise (TPP) suggests what could be the next change in the algorithm to make the next failing test pass. It was suggested by Uncle Bob here: https://blog.cleancoder.com/uncle-bob/2013/05/27/TheTransformationPriorityPremise.html

The most important thing in TPP is that it helps production code to change from specific to generic.

When we use TDD, our production code goes through a sequence of transformations. I used to think it was a transformation from stupid to intelligent. But I’ve begun to see that this is not the case at all. Rather, the code goes through a sequence of transformations from specific to generic.

Based on TDD Uncle Bob suggested to amend rules of TDD into the following form:

If we accept the Priority Premise, then we should amend the typical red-green-refactor process of TDD with the following provision:

*1) When passing a test, prefer higher priority transformations.

2. When posing a test choose one that can be passed with higher priority transformations.
3. When an implementation seems to require a low priority transformation, backtrack to see if there is a simpler test to pass.*

Below is the short cheatsheet for TPP.