[Jach]

I would expect that in the limit it would follow clean-room design (see https://en.wikipedia.org/wiki/Clean_room_design) -- you only have to do enough to prove to a court that you or someone you got to rewrite something according to your spec weren't sufficiently tainted by the original code to rise to copyright infringement (even if the result is bitwise identical due to primarily one obvious way to do it).

The GNU project offers a bit of guidance for what counts as "legally significant changes" to them: https://www.gnu.org/prep/maintain/maintain.html#Legally-Sign... They use about 15 lines rather than the GP's 10 lines, but point out there could be context where even many lines of repeated change (renaming a symbol) is not legally significant. My opinion is there's no hard line, like always you need to weigh the risk of getting sued (some contributors might even be unreachable or identifiable) against the cost of reimplementing even things legal counsel says are insignificant (at the end of which you still might get sued anyway and have to prove you did clean-room/changes were insignificant/it's fair use/whatever).