public static function getMutation(array &$tokens, $index)
    {
        $num = (float) $tokens[$index][1];
        if ($num == 0) {
            $replace = 1.0;
        } elseif ($num == 1) {
            $replace = 0.0;
        } elseif ($num < 2) {
            $replace = $num + 1;
        } else {
            $replace = 1.0;
        }
        $tokens[$index] = [
            T_DNUMBER,
            sprintf("%.2f", $replace)
        ];
    }