public function del(string $entity_pointer): void
    { 
        $valid_pointer = "/^([ug]):([a-f0-9\-]+):$/i";
        if(!preg_match($valid_pointer, $entity_pointer, $matches)) {
            throw new Exceptions\InvalidAclPointerException($entity_pointer, $valid_pointer, __FUNCTION__);
        }
        $type = $matches[1];
        $uuid = $matches[2];
        // check if uuid points to a actor or graph 
        // throw exception if $type and actual type don't match
        unset($this->permissions[$entity_pointer]);
        if($type=="g")
            unset($this->permissions_with_graph[array_search($uuid, $this->permissions_with_graph)]);
    }