Access control software must be based on a security policy model. Flaws in them may come from a lack of precision or some incoherences in the policy model or from inconsistencies between the model and the code. In this paper, we first present a formalisation of access control models based on the work on an algebra of security models by J. McLean (1988). Then, we describe the implementation of this framework and show how it can be used to obtain a particular security model: the Bell and La Padula security model. Last, as an example, we show how such a program can be integrated for secure databases. All our development is done within the Focal (Rioboo et al., 2004) programming environment which provides a language with object-oriented features allowing to write formal specifications, proofs and programs at the same level.