Its a virtual document: EnsLib.EDI.XML.Document, with the XML schema applied having been loaded into Ensemble via Ensemble -> InterOperate -> XML -> XML Schema Structures. 

Thanks @Enrico Parisi  and @Jeffrey Drumm - knew there'd be a way of dodging the limitation! 
 

Additional info: if we change the order of <allergies> and <patientNotes> in the schema, it does not change the order of elements produced in the XML.