title: Define a function count_down() that consumes a number a produces a string counting down to 1
body: <p>Define a function count_down() that consumes a number a produces a string counting down to 1.  </p>
  
  <pre><code>count_down(3) =&gt; "3, 2, 1"
  count_down(0) =&gt; ""  
  count_down(1) =&gt; "1"
  </code></pre>
  
  <p>Need help with these questions </p>