Retro-compatibility means previous Api version tests run on new Api version : 
* must fail when : 
- an api class is removed
- an api method is removed
- a parameter is added to an api method
- a parameter is removed from an api method
- a parameter is replaced
- a return parameter is replaced or removed
- class or method visibility is modified
- thrown exceptions are replaced by non descendant exception

* must success when : 
- a method is added
- a class is added
- a thrown exception is removed or replaced by a descendant exception 