UMLR — Integration between behavioral "sublanguages": Interactions and Activities

Although both Sequence Diagrams and Activity Diagrams have
added many advanced features in UML 2, these features appear to have been
added independently, so that they appear as two separate behavioral
languages. Consider improving the integration between them by supporting the
following:
a) Allow for an activity in an activity diagram to represent an invoked
operation in a message on a sequence diagram.
b) Allow for interaction diagram notations to be applied to activity
diagrams, such as the use of references, alternates, and gates.
c) Clarify that time and other constraints can be applied to an activity
diagram in a manner similar to how they are applied to sequence diagrams.