title: "Expression is not assignable" -- Problem assigning float as sum of two other floats in Xcode?
body: <p>In a piano app, I'm assigning the coordinates of the black keys.
  Here is the line of code causing the error.</p>
  
  <p>'blackKey' and 'whiteKey' are both customViews</p>
  
  <pre><code>blackKey.center.x = (whiteKey.frame.origin.x + whiteKey.frame.size.width);
  </code></pre>