Sequence Diagram - Create within an existing box

Question

I can't figure out how to create a participant within an already existing box.  When I do the following, It creates a second box "Computer1".

@startuml

box "Computer1"
participant ProcessA
end box

ProcessA -> ProcessA : Init

box "Computer1"
Create participant ProcessB
end box
ProcessA -> ProcessB : Spawn Process

@enduml

Answer 1

Not 100% sure if this is what you want:

@startuml
box "Computer1"
participant ProcessA

ProcessA -> ProcessA : Init

Create participant ProcessB

ProcessA -> ProcessB : Spawn Process

end box

@enduml

Comments

Yes, it absolutely does.  In my "real" diagram of course I have some other boxes, so I will need to organize my steps in terms of the boxes, not in the order the messages actually happen, but I think I can accomplish that by using the "order" option on messages.   Not sure if that makes sense, but yes, you answered my question!

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@plantuml Maybe there is a possibility to extend the box facility withe some sort of merge action.