An expert at your side will help avoid many problems: • he will help circumscribe the questions and will choose the precision level most adapted (beginners will have a tendency to built a heavy model with too many details), • he has a « flow vision », and will specify how to formulate the questions about the system, and how to transcribe the answers, • he will have a neutral point of view on modeling (will take nothing for granted), as much as on possible recommendations.