When, as a consequence of a user choice, a model becomes not feasible, a generic error is displayed "The model is in contradiction". It would be very useful (almost necessary) to have some additional explanations.
I would suggest:
1) Show the attributes that need to be modified in order to resolve the contradiction
2) Give the possibility to associate an user message to each constraint and, when the model is in contradiction, display the message(s) associated with the failing constraint rule(s)