Formalizing UML; Mission Impossible?
Peter van Emde Boas

Abstract:
Formalizing UML will lead to problems much larger than those 
encountered in formalizing ordinary computer related languages.
These problems concern not primarily the syntax. In fact that part is 
most likely to be formalizable at all. Instead I expect major 
problems with specifying a semantics as long as its purpose is 
undecided. Also the fact that UML is a hybrid of languages rather than 
a single one will make life difficult. Finally the fact that UML is 
visual yields unusual problems in the pragmatics of the language.