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As formal methods are increasingly used in the development of industrialscale software systems, there is a growing awareness of the need to integrate formal notations with existing structured software development methods. Such methods are good at managing the complexity of large systems, are well-known, well-documented and supported by CASE tools. This paper describes one method of using the Z notation together with Yourdon Structured Analysis to produce a requirements specification which is both structured and formal. The Entity Relationship model is used as the basis for a formal model of the system state. Each of the processes in the Data Flow model is formally specified using Z operation schema.s. The method is illustrated using a case study, the specification of data security in a secure computer system. The advantages and limitations of the integrated method are discussed, and directions for future work are outlined.

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