title: Swift NSArray mutation
body: <p>I have two variables one is mutable array second one is immutable.</p>
  
  <pre><code>let mutableArray = NSMutableArray(array: ["1","2","3"])
  
  let immutableArray: NSArray = mutableArray
  </code></pre>
  
  <p>But when I do append of the element to <code>mutableArray</code> my <code>immutableArray</code> is also changing.</p>
  
  <p>So my assumptions is to use <code>.copy</code> in <code>immutableArray</code> assigning. But is it optimal varian for solving of the issue? </p>