I. Introduction
When a program written in has not the expected behavior or is erroneous, the programmers look for the faults in their program. Indeed, they trust in the compiler. This latter has been largely tested because a fault in a compiler may spread over lot of programs since a compiler is used many times to justify the efforts relative to its development. Similarly, model transformations that form the skeleton of model based system development and so enable to generate code from high level model specifications have to be largely tested and trustworthy.