Observer::OnChange(nsIRDFResource* aSource,
Observer::OnChange(nsIRDFDataSource* aDataSource, nsIRDFResource* aSource,
Observer::OnChange(nsIRDFResource* aSource,                   nsIRDFResource* aProperty,                   nsIRDFNode* aOldTarget,                   nsIRDFNode* aNewTarget){    nsresult rv;    rv = rdf_WriteOp("chg-from", aSource, aProperty, aOldTarget);    if (NS_FAILED(rv)) return rv;    rv = rdf_WriteOp("chg-to", aSource, aProperty, aNewTarget);    if (NS_FAILED(rv)) return rv;    return NS_OK;}