How do you rewrite critical code safely? Carl and Richard talk to Phil Haack about his work on Scientist.NET. Phil talks about the challenges of rewriting code, which means taking something that works (more or less) and trying to make it better, at the risk of breaking it. Yes, test suites would help, but who has those on legacy projects? The Scientist library makes it easier for you to build a new version of a function in your legacy application and run it in parallel to the old function, so that you can validate the results in production without breaking anything. The conversation also turns to the value of rewrites, how tooling, techniques and technology have changed to make it more feasible to do major rewrites, and all the other geeky goodness that comes from hanging with Phil Haack. Happy episode 1400!