This paper provides an overview of how the Event-B language and verification method can be used to model and reason about system behaviour. Formal modelling and reasoning help to increase understanding and reduce defects in requirements specification. Sets and relations play a key role in modelling as do operators on these structures. Precise definitions and rules are provided in order to help the reader gain a strong understanding of the mathematical operators for sets and relations. While the emphasis is on mathematical reasoning, particularly through invariant proofs, the paper also covers less formal reasoning such as identification of problem entities supported by class diagrams and validation of formal models against informal requirements. The use of tools for animation, model checking and proof is also outlined.
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