n conforms to the <code>BidirectionalCollection</code> protocol,
<code>start</code> must be less than or equal to <code>end</code>.</p>

<p><strong>Parameters:</strong>
  <strong>start:</strong> A valid index of the collection.
  <strong>end:</strong> Another valid index of the collection. If <code>end</code> is equal to
    <code>start</code>, the result is zero.
<strong>Returns:</strong> The distance between <code>start</code> and <code>end</code>. The result can be
  negative only if the collection confo