title: How to use substringFromIndex in Playground?
body: <p>In the Swift playground (Xcode 7), why does the following</p>
  
  <pre><code>var someString = "my four word string"
  someString.substringFromIndex(advance(someString.startIndex, 5))
  </code></pre>
  
  <p>produce this error:</p>
  
  <blockquote>
    <p>'advance' is unavailable: call the 'advanceBy(n)' on the index</p>
  </blockquote>