 in <code>unicodeScalars</code>, the result of the initializer is <code>nil</code>.
For example, an attempt to convert the position of a UTF-8 continuation
byte returns <code>nil</code>.</p>

<p><strong>Parameters:</strong>
  <strong>utf8Index:</strong> A position in the <code>utf8</code> view of the <code>characters</code>
    parameter.
  <strong>unicodeScalars:</strong> The <code>UnicodeScalarView</code> instance referenced by both
    <code>utf8Index</code> and the resulting index.</p>

    <h4>Declarat