title: How can NSMutableArray add object using let in swift
body: <p>I created a NSMutableArray in swift using <code>let</code> and
  when I add addObject in the mutableArray then it will add it even though I
  used the <code>let</code> to assign a constant. Can anyone explain how <code>let</code> works in swift? If it doesn't allow you to add value in later then how is the following
  code working?</p>
  
  <pre><code>let arr : NSMutableArray = [1,2,3,4,5]
  arr.addObject(6)
  println(arr)
  </code></pre>