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In resolving the question of terminology around namespaces, we decided to define a new term 'absent'. The definition of that term is perilously close to the definition of 'undefined', surely they could be harmonized somehow? If not, the spec should articulate the distinction more clearly. On the subject of absent properties, we should also define which properties can be absent.
I think the intent was that "absent" should replace "undefined". The term "undefined" causes confusion because in many other specs it is used with a completely different meaning, to wit "not defined in this specification".
Undefined has been removed. We may still want to identify the properties that are allowed to be absent.