title: NSMutableSet vs NSMutableArray
body: <p>What's the difference?</p>
  
  <p>In my context, I need to be able to dynamically add to and remove objects. The user clicks on rows of a table that check on and off and thus add or remove the referenced object from the list.</p>
  
  <p>A wild guess is that array has indexed items while set has no indexes?</p>