In reviewing a book, I was reminded about the Gilded Rose kata.   It revolves around some legacy code that has no tests.   You need to make some changes to the code to support a new requirement.   The kata was created by Bobby Johnson (http://iamnotmyself.com/) and updated by Emily Bache (http://coding-is-like-cooking.info/).   

A detailed explanation of the Gherkin approach is in this PDF.   An html version is here. The code that the PDF references is at https://github.com/atdd-bdd/GildedRose .   There is one function that has not been refactored yet.  It’s left as an exercise to the reader.  

You can compare this solution to other solutions to the Gilded Rose kata to see alternative ways to approach this problem.