code: public void registerNamespace(String namespacePrefix, String namespaceUri) {
          if (_namespaces.containsKey(namespacePrefix))
              throw new RuntimeException("Namespace prefix '" + namespacePrefix + "' has already been registered");
          _namespaces.put(namespacePrefix, namespaceUri);
      }
docs: Registers a namespace for a given namespace prefix. This method must be called before registering any tags or attributes
  for that namespace. Note that extensions require namespaces to work properly.
  @param namespacePrefix the namespace prefix, cannot be null
  @param namespaceUri the namespace URI, cannot be null