  Sequence diagrams are a widely used design notation for describing software
behaviors. Many reusable software artifacts such as design patterns and design
aspects make use of sequence diagrams to describe interaction behaviors. When a
pattern or an aspect is reused in an application, it is important to ensure
that the sequence diagrams for the application conform to the corresponding
sequence diagrams for the pattern or aspect. Reasoning about conformance
relationship between sequence diagrams has not been addressed adequately in
literature. In this paper, we focus on required behavior specified by a UML
sequence diagram. A novel trace semantics is given that captures precisely
required behavior specified by a sequence diagram and a conformance relation
between sequence diagrams is formalized based on the semantics. Properties of
the trace semantics and the conformance relation are studied.
