Some models and modeling systems are designed in such a way that they allow additions to their structure. For example, OASIS (Operational Analysis and Simulation of Integrated Systems) is a software package designed to model river, reservoir, and hydropower systems to develop operating policies and optimize water use. OASIS has a GUI that allows easy configuration of the system. You can describe how the river system looks, where are the inputs and withdrawals, and enter historical data sets that the system is to work with. In addition, there is an Operation Control Language (OCL) - a special language to enter rules and constraints that are specific to your case study. OCL also acts as a bridge from OASIS to other computer programs. Users can express all operating rules as operating goals or operating constraints, and can account for both human control and physical constraints on the system. This takes care of all sorts of 'if-then' operations, which can go even beyond just operational rules. To model any system, one simply needs to approach the problem as a set of goals and constraints. The software then solves for the best means of moving water through the system to meet these goals and constraints. OCL allows data to be sent and received between OASIS and other programs, while the programs are running. Each program can then react to the information provided by the other. So you are dealing with a prefabricated 'closed' system, yet you have some flexibility to modify it to the particular needs of a study. There is clearly more flexibility than in case of a prepackaged model; however, the user is still operating within the set of assumptions and formalizations embedded in the model core of the software. There are also limitations in what OCL can handle as extensions to the OASIS system.
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