public function lowerCaseFirst()
{
$first = \mb_substr($this->str, 0, 1, $this->encoding);
$rest = \mb_substr($this->str, 1, $this->length() - 1, $this->encoding);
$str = \mb_strtolower($first, $this->encoding) . $rest;
return static::create($str, $this->encoding);
}