if (!mDataSource || !mContainer) return NS_ERROR_NOT_INITIALIZED;
RDFContainerImpl::AppendElement(nsIRDFNode *aElement){    NS_PRECONDITION(aElement != nsnull, "null ptr");    if (! aElement)        return NS_ERROR_NULL_POINTER;    nsresult rv;    nsCOMPtr<nsIRDFResource> nextVal;    rv = GetNextValue(getter_AddRefs(nextVal));    if (NS_FAILED(rv)) return rv;    rv = mDataSource->Assert(mContainer, nextVal, aElement, PR_TRUE);    if (NS_FAILED(rv)) return rv;    return NS_OK;}