Optional function parameter indication in UML Sequence Diagram

Question

Is there any formal way to signal that a function parameter is optional?

For example, in this image:

I want to inform that the nodes_clicked parameter could be populated or left empty.

Thanks.

Answer 1

Optional parameters are nothing handled in UML as it's jus "syntactic sugar" compilers implement to make coders happy. You can mimic that in a SD with a note attached to the message. However, I just would not go to that detail and leave it to the coder (they aren't children you have to tell each single step to take).