2.5.18. Soft variant
Constraint is a soft variant of constraint . Note that, from an academic point of view, a soft constraint is usually defined with a cost variable that quantifies how much the constraint is violated. We exceptionally breaks this rule when it seems to make sense from an application point of view. For instance, within the constraint, we reference the since it can be seen as a kind of relaxation of the constraint where we allow to use value 0 several times.