@Override
    public void startPrefixMapping(final String prefix, final String uri) throws SAXException {
        // Store prefix mappings and output them if we haven't seen the root
        // element yet.
        if (root == null) {
            prefixList.add(new StartPrefixMapping(prefix, uri));

        } else if (depth > 2) {
            // If we are deeper than the root element then treat prefix mappings
            // as normal.
            handler.startPrefixMapping(prefix, uri);
        }
    }
We want to suppress prefix mappings for first level elements that we see
again.
stroom-pipeline/src/main/java/stroom/pipeline/record/XMLRecordEmitter.java