title: Get NSIndexSet from NSArray
body: <p>NSArray has useful methods to find objects for specified indexes</p>
  
  <pre><code>// To find objects by indexes
  - (id)objectAtIndex:(NSUInteger)index
  - (NSArray *)objectsAtIndexes:(NSIndexSet *)indexes
  
  // To find index by object
  - (NSUInteger)indexOfObject:(id)anObject
  </code></pre>
  
  <p>However, I want to get <code>NSIndexSet</code> (multiple indexes) for given objects. Something like:</p>
  
  <pre><code>- (NSIndexSet *)indexesOfObjects:(NSArray *)objects
  </code></pre>
  
  <p>This method does not exist for <code>NSArray</code>. Am I missing something? Does someone know another standard method?  Otherwise I have to write this as a category method.</p>