Prior to release 4.0, LINGO allowed you to use primitive set names in the equations of a model. Primitive set names in a model's equations returned the index of the set member. Starting with release 4.0, LINGO requires you to use the @INDEX function to get the index of a primitive set member. If you would like to test your LINGO models from previous releases for instances where primitive set members appear in the model's equations, set CHKDUP to 1. Whenever you run a model, LINGO will issue an error message if duplicate names appear as set members and as variables in the model.