Optional function parameter indication in UML Sequence Diagram

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.

Solution

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).