One could see this in action in the Hellwig vs VMWare court case (one of the few GPL court cases). As a first step they had to show that Hellwig had made chang

One could see this in action in the Hellwig vs VMWare court case (one of the few GPL court cases). As a first step they had to show that Hellwig had made changes to code that VMWare uses that can be copyrighted. Especially difficult since every line in Linux is modified by a few different people. They failed at that and that ended the lawsuit if I remember correctly.