* Returns a prefix mapping containing all prefixes from the input model
* and from the configuration, with the configuration taking precedence.
*/
private PrefixMapping getPrefixes() {
if (prefixes == null) {
prefixes = new PrefixMappingImpl();
prefixes.setNsPrefixes(model);
for (String prefix: config.getPrefixes().getNsPrefixMap().keySet()) {
prefixes.setNsPrefix(prefix, config.getPrefixes().getNsPrefixURI(prefix));
}
}