title: Use of unresolved identifier 'numbers'
body: <p>I'm following Apple's - The Swift Programming Language iBook and I have to work with closures.</p>
  
  <pre><code>numbers.map({
     (number: Int) -&gt; Int in
     let result = 3 * number
     return result
  })
  </code></pre>
  
  <p>When I try to insert this piece of code (from the book) in XCode's playground, I get this: "Use of unresolved identifier 'numbers'"</p>