Yes, perhaps that's it. In that case, in my scenario I would not be good releasing my library as BSD. To my mind, writing a library according to a specificati

Yes, perhaps that's it. In that case, in my scenario I would not be good releasing my library as BSD. To my mind, writing a library according to a specification is not a derivative work of the specification. (I think a derivative work would be something like an annotated version of the specification, or a new specification that adopted parts of the original specification.) But like I say, I'm no expert on this so could very well be wrong.