UMLR — Section: 14.3.3

In sequence diagrams, the neg operator is used to describe invalid behaviours. However, people tend to interpret neg slightly differently depending on the context in which it appears, thus making it difficult to define a precise semantics for it. Two examples: A sequence diagram with a neg fragment is usually taken to describe also positive (valid) behaviours, i.e. the behaviours of the diagram with the neg fragment simply omitted. This implies that the empty trace should be positive for the neg fragment in this context. Another common use of neg is to state that one of the alternatives (operands) of an alt construct describes the invalid behaviour. In this case, the neg fragment has no positive behaviours (not even the empty trace). Recommendation: Consider introducing another operator in addition, due to the different uses of the neg operator.