You may need to test in the condition part of a rule whether an event attribute is present or not. Indeed, the structure of events is flexible, and even for a given event type, the content of its context may vary from one instance to the other. The following test detects the presence of an attribute (here, named "attr"), regardless of its value:
The condition: (EVT_1.attr = nil) is true only if the attribute "attr" is not part of the context of the event EVT_1.
The condition: (EVT_1.attr != nil) is true only if the attribute "attr" is part of the context of the event EVT_1, regardless of its value.
Do not confuse the keyword nil here with the null value of the event attribute.
The condition: (EVT_1.attr = nil) is true only if the attribute "attr" is not part of the context of the event EVT_1.