Honest question - where does one draw the line between what is protected under copyright and what isn't? You mentioned "taxonomies" - what does that mean exactly? Does a single package with one class and one method count? What about two?
The place the line ought to be is where there is only one way to do a particular thing. If the function signature has to be a specific one to achieve compatibility then it isn't a creative act to choose it, it's chosen for you as a functional requirement. You may not even want the interface to look like that, but it's the only way to achieve compatibility.
The implementations are something else entirely, specifically because there are a thousand ways to implement the same function that are all functionally equivalent but nonetheless have different code. It's not a matter of how long it is, it's a matter of whether there is another way to do it.
