@Override
    public void startPrefixMapping(final String prefix, final String uri) throws SAXException {
        super.startPrefixMapping(prefix, uri);

        cb.append(START_PREFIX_MAPPING);
        cb.append(prefix);
        cb.append(DELIMITER);
        cb.append(uri);

        writeln(cb);
    }
@param prefix the Namespace prefix being declared. An empty string is used
              for the default element namespace, which has no prefix.
@param uri    the Namespace URI the prefix is mapped to
@throws org.xml.sax.SAXException the client may throw an exception during processing
@see #endPrefixMapping
@see #startElement
stroom-pipeline/src/main/java/stroom/pipeline/filter/TestSAXEventFilter.java