GPL - GSL and derivative work

[Edwin Robert Tisdale]

Mark Galassi wrote:

> Once you reimplement GSL, you can say that
> the software does not depend on GSL for its functioning.
> 
> But until you do, if you distribute software and say,
> "This will only work with GSL
>  because no other implementation of that API exists."
> then you have a derivative work.

Suppose that the court agrees with you, counselor,
then someone reimplements the GSL API.
Would we still have a derivative work?
Would other applications using the GSL API
but written after the GSL API was reimplemented
then also be derivative work?