title: Merge node in activity diagram
body: <p>I'm learning Activity diagram in UML and now I'm confused with the following diagram for representing procedures when calling a pizza.<a href="https://i.stack.imgur.com/tbR8B.png" rel="noreferrer"><img src="https://i.stack.imgur.com/tbR8B.png" alt="enter image description here"></a></p>
  
  <p>I think a merge node should be added before the join node because if not, the join node will wait forever. Is that correct?
  Thank you.</p>