code: public void addSceneObject(int key, GVRSceneObject sceneObject) {
          // only add if not present
          if (!auxSceneObjects.containsKey(key)) {
              auxSceneObjects.put(key, sceneObject);
              handSceneObject.addChildObject(sceneObject);
          }
      }
docs: Add a {@link GVRSceneObject} to this {@link IOHand}
  
  @param key         an int value that uniquely helps identify this {@link GVRSceneObject}.
  So that
  it can easily be looked up later on.
  @param sceneObject {@link GVRSceneObject} that is to be added.