code: public static boolean set(RythmEngine engine) {
          if (_engine.get() == null) {
              _engine.set(engine);
              return true;
          } else {
              return false;
          }
      }
docs: Set the engine instance to a {@link ThreadLocal} variable, thus it is easy to
  {@link #get() get} the current
  <code>RythmEngine</code> dominating the rendering process.
  <p/>
  <p><b>Note</b>, this method is NOT an API to be called by user application</p>
  
  @param engine
  @return {@code true} if engine instance set to the threadlocal, {@code false}
  if the threadlocal has set an instance already