code: public static ActivityEngine getDefaultEngine() {
  		if (defaultEngine == null) {
  			synchronized (ActivityEngineFactory.class) {
  				if (defaultEngine == null) {
  					defaultEngine = makeDefaultEngine();
  				}				
  			}
  		}		
  		return defaultEngine;
  	}
docs: Retrieves the currently registered instance of engine, as a default.
  
  @return
  the currently registered instance of engine, as a default.