UMLR — UML2-rtf issue: communication diagram

UML2-rtf issue related to communication diagrams:

Section 14.4 describes the sequence expression tied to messages on a communications diagram. However, the abstract syntax does not (as far as I can trace the diagram) model the sequence expression. This leaves the sequence expression semantics wide open.

Some tools implement the sequence express but the concept of threads and nesting implicit in the expression syntax are undefined in the meta model.

Recommendation: Add a SequenceExpresion element into the model associate it with message, and define it explicitly. Describe the semantics of nesting messages.