code: public GrProperty addProperty(String name, Object value) {
  		GrProperty prop = GrAccess.createProperty(name);
  		prop.setValue(value);
  		return getPropertiesContainer().addElement(prop);
  	}
docs: add a new property, throw a RuntimeException if the property already exists
  @param name of the property
  @param value of the property
  @return the added property