Yes, this is interesting as "=>" and "," are not interchangeable. The net of this is that this question is one heck of a lot more complex than it sounded at first!

If thing X is described as a "delta" from thing Y ("but"). Is Y well defined? And is it possible to clearly understand what X will do even given that you understand Y? (e.g what the "but" delta means).

As another thought, if defining the complete behavior is too complex, is there some subset that can be described that would be generally useful?