title: How to increase String.Index in swift3?
body: <p>In swift2.3
  ++ operator use for string.index increase</p>
  
  <p>eg.
  i++</p>
  
  <p>I changed to swift 3 that code happen 
  "
  Unary operator '++' cannot be applied to an operand of type '@lvalue String.Index' (aka '@lvalue String.CharacterView.Index') " in swift 3</p>
  
  <p>I rewrite 
  eg.
  i += 1</p>
  
  <p>but this code can't solve.
  Please Help me.</p>