Feature Request - Children

I was most perplexed when I wrote:

``` scss
ul > li:exactly( 3 ) img { opacity: 0.5; }
```

And found it didn't work... I would expect the result to be:

``` css
ul > li:nth-last-child(4):first-child, 
ul > li:nth-last-child(4):first-child ~ li img {
    opacity: 0.5;
}
```

Is there any intent to implement this? are you accepting PRs ? :)
