code: public void registerTag(String namespacePrefix, String tagName, Class<?> clazz, String fieldName, Class<?> fieldClass) {
          if (!_namespaces.containsKey(namespacePrefix))
              throw new RuntimeException("Namespace prefix '" + namespacePrefix + "' has not been registered yet");
          _xstream.alias(namespacePrefix + ":" + tagName, fieldClass);
          _xstream.aliasField(namespacePrefix + ":" + tagName, clazz, fieldName);
          _tags.computeIfAbsent(namespacePrefix, k -> new HashSet<>()).add(tagName);
      }
docs: Registers a tag corresponding to a specific field of a specific class, in a the given namespace.
  @param namespacePrefix namespace prefix, required
  @param tagName tag name, required
  @param clazz class containing the field, required
  @param fieldName field name, required
  @param fieldClass field type, required