Adds a string property.
Previous values for the same property are kept.
This method is meant for RDF properties allowing multiple values, see [nrl:maxCardinality](nrl-ontology.html#nrl:maxCardinality).
This method corresponds to [xsd:string](xsd-ontology.html#xsd:string).
| this |
the `TrackerResource` |
| property_uri |
a string identifying the property to modify |
| value |
the property object |