title: Why is this string immutable?
body: <p>Why does the code give the error - Attempt to mutate immutable object with appendFormat: ?</p>
  
  <pre><code>NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
  
  for (NSTextTestingResult *match in matches) {
      &lt;omitted&gt;
      NSMutableString *value;
      value = (NSMutableString *)[response stringWithRange:range];
      if ([dict objectForKey:@"traveler"])
          [dict objectForKey:@"traveler"] appendFormat:@"%@", value];     //  Errors here
  
  [dict setObject:value forKey:key];
  }
  </code></pre>
  
  <p>Value is being created as a <code>_NSCFString</code>.</p>