Cleanroom is an overstatement. The reason people do cleanroom is to eliminate risk of liability, it's simple, and it's foolproof. You can't mess it up. It also

Cleanroom is an overstatement. The reason people do cleanroom is to eliminate risk of liability, it's simple, and it's foolproof. You can't mess it up. It also eliminates other potential issues (like tortious interference with someone who signed an NDA). Legally, it's overkill. It's like using a thermonuclear bomb to swat a fly. If you're going up against Microsoft, overkill absolutely makes sense; you might be legally right, but it will take millions of dollars in legal fees to show that. Programmers are cheaper than lawyers. In the general case, though, you can just rewrite the old code and be done with it. Yourself. Looking at the old code too, so long as you don't borrow it. You want to be a little bit careful since it's not just the literal words (you can break copyright on a novel if you rewrite the same plot in your own words; same thing with code), but "a little careful" suffices. It's a bit harder to transform a piece of code into something non-infringing, rewriting section-by-section, but most lawyers say that's not impossible either. (Of course, clearly, none of that happened in this case)