title: Removing a hash element if its value is "undef"
body: <p>I have seen <code>delete</code> that deletes a hash element. But what if some of the elements dates have <code>undef</code> for a value?</p>
  
  <p>I would like to delete all hash elements whose value is <code>undef</code>.</p>
  
  <p>How would I do that in a single line, or the simplest way.</p>