Object-oriented programming, nested models, information systems, simulation

The paper is oriented to general technique of the synthesis of two levels existing in information systems and in larger ones encompassing them. The technique is related to formal descriptions and to simulation of such systems. One level consists in abstract information processing and the other in existing in time. Expected changes of such systems may concern only one of the levels. The economy and ergonomics of the description and simulation is supported by model nesting, namely that the information processing model is nested in the simulation model of the physical system. The optimal way is applying programming languages that are object-oriented, block-oriented and process-oriented.