public function insert($substring, $index) {
		if ($index <= 0) {
			return $this->prepend($substring);
		}

		if ($index > $this->length()) {
			return $this->append($substring);
		}

		$start = mb_substr($this->string, 0, $index, $this->encoding);
		$end = mb_substr($this->string, $index, $this->length(), $this->encoding);
		return new Text($start . $substring . $end);
	}