code: function(item){
      var children = item.children;
  
      if(typeof item.last === "function") return item.last();
      if(typeof item.nodeType === "number") return item;
  
      if(children && children.length) return combine.last(children[children.length - 1]);
      if(item.group) return combine.last(item.group);
  
    }
docs: get the last dom in object(for insertion operation)