2619.js

// ...

var Anonymous = less.tree.Anonymous,
    Selector  = less.tree.Selector;

"parent-selectors": function() {
    var frames = this.context.frames,
        i = frames.length,
        elements = [];
    while (i-- > 0)
        if (frames[i].selectors && frames[i].type === "Ruleset")
            for (var j = 0; j < frames[i].selectors.length; j++)
                for (var k = 0; k < frames[i].selectors[j].elements.length; k++)
                    elements.push(frames[i].selectors[j].elements[k]);
    return new Anonymous((new Selector(elements)).toCSS().slice(1));
}