title: Scene Kit Swift Node Multiple Rotation
body: <p>In SceneKit with Swift, I can make only one rotation.</p>
  
  <p>I want to rotate camera at the z-axis every time the user taps the screen.</p>
  
  <pre><code>func handleTap(){
      println("start")
      SCNTransaction.begin()
      SCNTransaction.setAnimationDuration(1.0)
  
      SCNTransaction.setCompletionBlock(){
          println("done")
      }
  
      _cameraNode.rotation = SCNVector4Make(0, 0, 1, CFloat( -M_PI_4 ) * 0.5 )
  
      SCNTransaction.commit()
  
  }
  </code></pre>
  
  <p>However, I want to keep rotating it with every touch. I think I need a <code>scnvector4</code> multiplier, but I couldn't find that method.</p>
  
  <p>How can I multiply 2 <code>scnvector4</code>s or what's another way I can keep rotating z-axis ?</p>