1. A computer system for implementing derived union constraints in a target code of a programming model, the programming model using a modeling language, the computer system comprising: given the modeling language is of the Unified Modeling Language (UML) type, using the modeling language, defining metaclasses and having model elements as instances of metaclasses, model elements including properties having respective values, and UML-derived-union-constraints being distinct from stereotypes and being constraints on model elements defined by a superset of values of a respective property; modeling means configured for providing a model element having one or more derived union properties, each derived union property having a superset of values denoted by the property; means configured for tracking UML-derived-union-constraints from the one or more derived union properties of the model element including keeping track of derived union items of the model element; an interpreter for interpreting the tracked UML-derived-union-constraints and generating target code therefrom that implements and enforces the UML-derived-union-constraints, including generating code implementing overriding operations on classes that contain subletting properties for the one or more derived union properties of the model element, wherein the means for tracking provides to the interpreter indications of which properties the operations represent a superset; and output means outputting the generated target code to a user.