As it is often the case with open projects, we develop through successive iterations. While reflection and modelling steps are needed before the proper development, we know by experience that expansive specifications do not guarantee an adapted solution. We usually lose in the depth of uses reality what we gain in theoretical modelling. That is why we privilege a form of development in which we quickly create an application prototype and give it to the users who will be able to give more precise specifications. The users feedback enables us to provide a more elaborated prototype, and so on, until reaching the final application. Computer projects tend to reach users too late, when the development stage only allows for esthetic modifications. This is precisely what we try to avoid.