A major hindrance to the use of formal methods is the difficulty to validate the models, particularly at the early stages of the development. We propose to build simulations: programs automatically generated from the specifications but with user-provided implementations of the non-executable traits of the models. We present such a simulation. Of course, the question of the fidelity of the simulation to the model is raised in such a setting. We provide a formal definition of fidelity and the proof obligations that can be attached to each hand-coded element so that fidelity can be proven.