how can i move two main nodes that have no targets ?

[khashabawy]

In the attached file how can i move Plastics and Electronics :

for moving any other sub nodes i do this :

        $cat = $this->nested_set->getNodeFromId($cat_id);

        if($cat["cat_parent_id"]){

            $parent = $this->nested_set->getNodeFromId($cat["cat_parent_id"]);

            if($direction == "up"){                            
                $this->nested_set->setNodeAsFirstChild($cat , $parent);
            }else{                
                $this->nested_set->setNodeAsLastChild($cat , $parent);
            }       
        }

I there any way to :

1- move nodes by one step ( not as first child or last child ) ?
2- move main nodes that have no targets (parents) ?

[quasiperfect]

use setNodeAsNextSibling or setNodeAsPrevSibling it works for me