While I empathize with the feeling of "I rewrote or tossed out so much, the original copyright and license don't matter"... that's not the right criteria. Instinctively we know that some code isn’t really worth it copyright-wise. There is a criteria. But it’s got nothing to do with % of code. If you rewrote 1% or 99%, you still have to consider… https://en.wikipedia.org/wiki/Abstraction-Filtration-Comparison_test It’s a non-trivial process, so generally speaking if you’re using someone else’s code, just take/keep their copyright notice, and respect their license.