An extensible formal semantics for UML activity diagrams
  This paper presents an operational semantics for UML activity diagrams. The purpose of this semantics is three-fold: to give a robust basis for verifying model correctness; to help validate model transformations; and to provide a well-formed basis for assessing whether a proposed extension/interpretation of the modeling language is consistent with the standard. The challenges of a general formal framework for UML models include the semi-formality of the semantics specification, the extensibility of the language, and (sometimes deliberate, sometimes accidental) under-specification of model behavior in the standard. Our approach is based on structural operational semantics, which can be extended according to domain-specific needs. The presented semantics has been implemented and tested. 
Zamira Daw and Rance Cleaveland
cs.LO