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An access control design can be viewed as a three layered entity: the general access control model; the parameterization of the access control model; and the initial users and objects of the system before it goes live. The design of this three-tiered mechanism can be evaluated according to two broad measures, the expressiveness versus the complexity of the system. In particular, the question arises: What security properties can be expressed and verified? We present a general access control model which can be parameterized at the second layer to implement (express) any of the standard Discretionary Access Control (DAC) models. We show that the safety problem is decidable for any access control model implemented using our general access control model. Until now, all general access control models that were known to be sufficiently expressive to implement the full range of DAC models had an undecidable safety problem. Thus, given our model all of the standard DAC models (plus many others) can be implemented in a system in which their safety properties are decidable.