Assert and negate revisited: Modal semantics for UML sequence diagrams

David Harel*, Shahar Maoz

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

115 Scopus citations


Live Sequence Charts (LSC) extend Message Sequence Charts (MSC), mainly by distinguishing possible from necessary behavior. They thus enable the specification of rich multi-modal scenario-based properties, such as mandatory, possible and forbidden scenarios. The sequence diagrams of UML 2.0 enrich those of previous versions of UML by two new operators, assert and negate, for specifying required and forbidden behaviors, which appear to have been inspired by LSC. The UML 2.0 semantics of sequence diagrams, however, being based on pairs of valid and invalid sets of traces, is inadequate, and prevents the new operators from being used effectively. We propose an extension of, and a different semanticsfor this UML language - Modal Sequence Diagrams (MSD ) - based on the universal/ existential modal semantics of LSC. In particular, in MSD assert and negate are really modalities, not operators. We define MSD as a UML 2.0 profile, thus paving the way to apply formal verification, synthesis, and scenario-based execution techniques from LSC to the mainstream UML standard.

Original languageEnglish
Pages (from-to)237-252
Number of pages16
JournalSoftware and Systems Modeling
Issue number2
StatePublished - May 2008
Externally publishedYes


FundersFunder number
John von Neumann Minerva Center for the Development of Reactive Systems
Weizmann Institute of Science
Israel Science Foundation287/02-1


    • Formal semantics
    • Livesequence charts
    • Sequence diagrams
    • Standardization
    • UML Interactions


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