@Override
    public void startDocument() throws SAXException {
        // Only write one start document event.
        if (!started) {
            started = true;

            super.startDocument();
        }
    }
@throws SAXException Not thrown.
@see stroom.pipeline.filter.AbstractXMLFilter#startDocument()
stroom-pipeline/src/main/java/stroom/pipeline/filter/MergeFilter.java