ionalCollection</code> protocol.
<strong>Returns:</strong> An index offset by <code>n</code> from the index <code>i</code>. If <code>n</code> is positive,
  this is the same value as the result of <code>n</code> calls to <code>index(after:)</code>.
  If <code>n</code> is negative, this is the same value as the result of <code>-n</code> calls
  to <code>index(before:)</code>.</p>

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