title: ERROR for attempting mutating immutable object
body: <p>What is this error? I am using mutable type but still it is not working!</p>
  
  <p>ERROR IS :   </p>
  
  <blockquote>
    <p>Terminating app due to uncaught exception
    'NSInvalidArgumentException', reason: 'mutate immutable
    object with deleteCharactersInRange:'</p>
  </blockquote>
  
  <pre><code>NSMutableString *line= [NSMutableString stringWithString:@"here is the string"];
  [line deleteCharactersInRange: NSMakeRange(1,9 )];
  </code></pre>