title: Algorithm to return all combinations of k elements from n
body: <p>I want to write a function that takes an array of letters as an argument and a number of those letters to select. </p>
  
  <p>Say you provide an array of 8 letters and want to select 3 letters from that. Then you should get:</p>
  
  <pre><code>8! / ((8 - 3)! * 3!) = 56
  </code></pre>
  
  <p>Arrays (or words) in return consisting of 3 letters each.</p>