title: Swift "Cannot invoke '-=' with argument list of type '(@lvalue CGFloat, $T7)'
body: <p>Can someone please explain to me why I am getting this error message when I try to run this code:</p>
  
  <pre><code>let sprite = SKSpriteNode(imageNamed:"Spaceship")
  
  sprite.xScale = 0.5
  sprite.yScale = 0.5
  sprite.position = location
  
  self.addChild(sprite)
  
  var dt: Float = 1.0/60.0
  
  sprite.position.x -= 100.0*dt;
  </code></pre>
  
  <p>The error message appears on the last line by the way</p>