Here the term "instance" refers primarily to instance data. Instance data is persistent data allocated to enable code reentrancy. It is commonly found in shared libraries, as well as in instances of a class in object oriented programming (OOP). The programming model presented here is similar to OOP, but with a very different emphasis. The OOP model along with its terminology is derived from SIMULA, a simulation programming language. Most programming is not simulation, so the fit is somewhat awkward. The encapsulation and information hiding provided by SIMULA can be used in general programming, without the need to impose the entire simulation model.