docs: function getLast(isNew, opt_key){
      opt_key = opt_key || 'selectors';
      var items = this.get(opt_key);
      var len = items.length;
  
      if (isNew){
        len++;
        items.push([]);
  
        var nest = this.get('nest');
        var nLen = nest.length;
        if (nLen > 1){
          //console.log([nest, nest[nLen - 2], len - 1]);
          //push the new rule to is father
          items[nest[nLen - 2]].push(items[len - 1]);
        }
      }
  
      return items[len - 1];
    }