public void removePrefix(final String prefix) {
        final String namespaceURI = prefixToUriMap.remove(prefix);

        // Remove from URI to prefix map.
        if (namespaceURI != null) {
            final Set<String> prefixSet = uriToPrefixMap.get(namespaceURI);
            if (prefixSet != null) {
                prefixSet.remove(prefix);
                if (prefixSet.isEmpty()) {
                    uriToPrefixMap.remove(namespaceURI);
                }
            }
        }
    }
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