title: Semantics of execution occurrence in sequence diagrams
body: <p>I am teaching a course that uses UML as the tool to model systems. In the course we teach sequence diagrams and while preparing the course I found that there is no clear definition of the semantics of an "execution occurrence". 
  For example. If an object makes a self call "wait for input" and this call is inside an execution occurrence, can the object receive messages? Obviously I think yes, because it is waiting for messages.</p>
  
  <p>But this counters the idea that execution occurrences mean that the object is currently "busy" or "active".</p>
  
  <p>Any help/ideas? My conclusion was to leave the subject and tell the students to disregard execution occurrences. Any better ideas?</p>