Difference log production for model merging

Methods and apparatus are provided for production of a difference log in a data processing system. The difference log defines differences between process models defined in system memory. For each of the process models, model structure data provided in memory defines a hierarchy of SESE regions representing the structure of that model. Also provided in memory are model comparison data defining correspondences between elements of the models, and region comparison data defining correspondences between regions of the SESE region hierarchies for the models. The model comparison and region comparison data are analyzed to identify differences between the SESE region hierarchies, and a difference log defining said differences is produced. In preferred systems, the model structure data and the region comparison data are computed for the models, and the difference log has a hierarchical structure corresponding to the structure of the process models.