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Once a model is defined using a selected software tool, the model must generally be debugged to ensure that it works correctly. The process of demonstrating that a model works as intended is referred to in simulation literature as model verification. It is much easier to debug a model built in stages and with minimal detail than to debug a large and complex model. Eliminating bugs in a program model can take a considerable amount of time, especially if a general purpose programming language (e.g., C++) in which frequent coding errors occur is used. Most simulation languages provide a trace capability in the form of audit trails, screen messages, graphic animation, or a combination of all three. A trace enables the user to look inside of the simulation to see if the simulation is performing the way it should. Good simulation products provide interactive debugging capability which further facilitates the debugging process. A thorough "walk-through" of the model input is always advisable.