Audience member asks whether mutation testing is appropriate for every project/client, and whether to focus on core domain/business logic first. Schirp's answer: yes to every project — human time is the most expensive resource and Mutant takes ~10–20 seconds per subject. And no to restricting to core domain — any line of code can blow up in production (even a rendering glitch shows on every page), so trying to find the 'perfect place' to mutation-test is fruitless, especially in Ruby where everything can kill everything. Recommends running it everywhere because it is cheap compared to human time.