IfcTimeSeries.HasExternalReference inverse lower bound 1

[aothms]

I was working on some sample data and realized that the lower bound for HasExternalReference on IfcTimeSeries is 1, so an external reference for time series is required to exist. Is that intentional?

HasExternalReference: SET [1:?] OF IfcExternalReferenceRelationship FOR RelatedResourceObjects

Reference to an external reference, e.g. library, classification, or document information, that is associated to the IfcTimeSeries.

http://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcTimeSeries.htm

[Moult]

Good point, I am working with Brickschema who has a lot of experience in referencing timeseries (they require an external reference) so once I have a prototype testing this out in a few scenarios that might help inform this answer.