title: How to plus "NSString" in IOS programming?
body: <p>How to plus <code>NSString</code>?
  Example: </p>
  
  <pre><code>NSString *a=@"hello";
  NSString *b=@"world";
  NSString *s=@"";
  </code></pre>
  
  <p>I want: </p>
  
  <pre><code>s = "helloworld";
  </code></pre>
  
  <p>Thank for helping.</p>