<code>endIndex</code> or
before the <code>startIndex</code> of this collection.</p>

<p><strong>Parameters:</strong>
  <strong>i:</strong> A valid index of the collection.
  <strong>n:</strong> The distance to offset <code>i</code>. <code>n</code> must not be negative unless the
    collection conforms to the <code>BidirectionalCollection</code> protocol.</p>

<p><strong>See Also:</strong> <code>index(_:offsetBy:)</code>, <code>formIndex(_:offsetBy:limitedBy:)</code>
<strong>Complexity:</strong> O(1) if the