+ DataElement data_element;
+ data_element.data.iov_base = const_cast<char *>(existing_data.data());
+ data_element.data.iov_len = existing_data.size();
+ data_element.suitable_for_stream_start = NOT_SUITABLE_FOR_STREAM_START; // Ignored by add_data_raw().
+
+ vector<DataElement> data_elements;
+ data_elements.push_back(data_element);
+ add_data_raw(data_elements);