docs: public static synchronized void unregister(final String serviceName,
                                                 final Callable<Class< ? >> factory) {
          if ( serviceName == null ) {
              throw new IllegalArgumentException( "serviceName cannot be null" );
          }
          if ( factories != null ) {
              List<Callable<Class< ? >>> l = factories.get( serviceName );
              if ( l != null ) {
                  l.remove( factory );
              }
          }
      }