UML2 — Activity Diagrams: Relax Traverse-to-Completion semantics

In my interpretation of the current semantics description of UML
activity diagrams (Superstructure, Final Adopted Spec, ptc/03-08-02) I
have identified some rather unpleasant properties of the current
traverse-to-completion semantics. The full discussion together with
examples can be found in the attached .pdf, the short of it is:

*) the current semantics does not prevent deadlocks (as it is
supposed to do)

*) it rather induces deadlocks even in simple examples (e.g. examples
in the UML spec are wrong)

*) it makes for a very complex evaluation and introduces unnecessary
synchronization in the (basically asynchronous) notation of Activiy
Diagrams.

I therefore propose to relax the semantics of token flow by dropping
the constraint that every Action has to accept all tokens for all its
input pins at once. MergeNodes should als be able to buffer tokens
until their conditions are satisfied. This is a more natural way of
interpreting ADs.

This is a duplicate of issue 7221