How to extract attributes from inside transitions?

How-to Guides
1 
package Demo {
    state def A_behavior {
        attribute def GoOnline {}
        attribute def GoOffline {}

        entry;
        then OFF; // defines OFF as initial state
        state OFF {}
        state ONLINE {}

        transition off_to_online
            first OFF
            accept GoOnline // GoOnline is the event
            then ONLINE;
        transition online_to_off
            first ONLINE
            accept GoOffline // GoOffline is the event
            then OFF;
    }
}

I am not able to extract the GoOnline attribute from the off_to_online transition. I am able to get the AcceptActionUsage with the name of accepter, but the payload_argument is empty. Does anyone know where could I find payload_argument? Thanks.

2

It’s because you want payload_parameter, and it’s FeatureTyping:

payload_argument is payload_parameter.feature_value_expression

3

Is this SysIDE-specific, or is it SysMLv2-wide thing?

4

SysML