| xades4j.xml.unmarshalling.QualifyingPropertiesDataCollector | 
Passed to a QualifyingPropertiesUnmarshaller to collect the property
 data obejcts. This is used instead of a collection that is returned by the unmarshaller
 because it allows controlling the number of occurrences of each property.
 
 All the methods will throw PropertyTargetException if an attempt is made
 to add a repeated instance or an instance of a type that was already added and
 should occur only once. Also, NullPointerException is thrown if an attempt
 is made to add a null reference.