This is a hypothetical question, but I am curious. Feel free to ignore it, when it seems too theoretical for you.
Some people may know the situation that you implement a feature and in the end you have added like 10 lines and removed 100 and from the 10 there are possibly 5 that just were moved around and not really added.
Now suppose Alice writes code and releases it with a strong copyleft, e.g., using the GPL. Then Bob contributes to this code and improves its efficiency by removing redundant (but not dead) code and in the end Bob contributed a lot of deletions.
Afterward, Alice wants to re-license the code with a weak copyleft, e.g., using the BSD license. To do so, she asks every contributor to re-license their code and removes any code from persons that cannot be reached or not willing to re-license.
Let's say Carol is rewriting the missing pieces and licensing them under the BSD license, so there is no conflict with Alice already knowing the removed code.
Now let's assume the problem, that Bob did not re-license his changes, but contributed a lot of deletions. The deletions made a significant difference, so you would think that copyright should apply. At least for the diff (containing the copyrighted content to be removed) it certainly does.
How does copyright apply to this case to the resulting code (without its commit history) and what can Alice do about the GPLed deletions? You could say she cannot keep them, as then the project stays the same as after the work of Bob. But should she re-add the deleted code?
Even when she does and Carol removes it again, the result is the same and looks just like the code with the GPLed deletions, so it cannot be free of GPL work (i.e. the negative code).
