title: Swift error : value of optional type 'Double?' not unwrapped
body: <p>I am newbie in Swift, what is this error : </p>
  
  <pre><code>let lvt=self?.lastVibrationTime
  let delta=self!.deltaTime
  let sens=self!.shakeSensitivity
  let time:Double = CACurrentMediaTime()
  
  //error is on `lvt` and says : Error:(37, 27) value of optional type 'Double?' not unwrapped; did you mean to use '!' or '?'?
  if time - lvt &gt; delta &amp;&amp; data.userAcceleration.x &lt; sens {
                      println("firmly shaken!")
                      self?.vibrateMe()
                  }
  </code></pre>