code: public Set<String> getAllowedTagsForNamespacePrefix(String namespacePrefix) {
          if (namespacePrefix == null)
              return _defaultTags;
          return _tags.get(namespacePrefix);
      }
docs: Returns the tags allowed for a given namespace prefix (null means the default namespace).
  @param namespacePrefix namespace prefix
  @return set of allowed tags, maybe empty but never null