@Override
    public Iterator<String> getPrefixes(final String namespaceURI) {
        final Set<String> prefixSet = uriToPrefixMap.get(namespaceURI);
        return NullSafe.isEmptyCollection(prefixSet)
                ? null
                : prefixSet.iterator();

    }
Holds the namespaces and their prefixes encountered in the output of the element being stepped
stroom-pipeline/src/main/java/stroom/pipeline/filter/NamespaceContextImpl.java