it is about copying of interface definitions
Yes, which were subject to copyright. But you can write as many MULTI(X, Y) functions as you like; copyright isn't in a single function signature (or even a list). You can add MULTI(X,Y) to every language you create. This case is not about what most developers do, or even library software vendors.
You just can't copy Java's API source code and make it your own (unless you accept the GPL, or promise to be 100% compatible).
Biting the hand that feeds IT
