Consider now one such software product, say MS Word for Mac. Starting with version V1, which denotes the specific software system constituting the software prod- uct at the time of its first release, this product will suffer a number of possible changes in its constituents in order to fix bugs, to include new functionalities, to improve per- formance, security, precision, etc. Each of these changes leads to distinct code, but some of them (those that are not just bug fixings) will also lead to a new program, while others (those that concern changes in the external interface) will also lead to a distinct software system, namely to a different version (V2) of the same product.