title: Why won't substrings work in Swift 4 anymore?
body: <p>one of the pods in my project won't compile and I can't figure out why. Truthfully, I've never had to deal with <code>String.UnicodeScalarView</code> so I have no idea what that is and the docs aren't being very helpful. They are still using <code>string[subscript]</code> in the Swift 4 documentation but Xcode is complaining about <code>subscript</code> being obsolete. Any ideas?
  <a href="https://i.stack.imgur.com/PWtKt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PWtKt.png" alt="enter image description here"></a></p>