1. In a digital data processing system operable in a state-free manner, of a type having an output generator (90) for generating an output manifestation (100) of a model manipulated by the digital data processing system, an object generator (30) for generating objects (250) describing the model, and an object memory (60) for storing the objects (250), an improvement comprising: means for generating a field within a given object (250) for storage of an error reference (260); an error generator means for generating an error object (300) containing error details for that object (250) and storing the error object (300) in an error memory (70); means for providing the error reference (260) to the object generator (30),the error reference (260) providing identifying information for the error object (300); and mean for storing a null reference (270) in the field of a generated object if no error occurs during the generation of the object.