public void ComputeInAlternatives(KAOSModel model)
        {
            this.model = model;

            foreach (var goal in model.RootGoals()) {
                goal.InSystems = new HashSet<AlternativeSystem> (model.AlternativeSystems());
                DownPropagate (goal);
            }

            foreach (var g in model.RootGoals())
                Simplify (g);
        }