protected function getAcl($depth = 0)
 {
     static $id = 1;
     $acl = new Acl($id, new ObjectIdentity($id, 'foo'), $this->getPermissionGrantingStrategy(), array(), $depth > 0);
     // insert some ACEs
     $sid = new UserSecurityIdentity('johannes', 'Foo');
     $acl->insertClassAce($sid, 1);
     $acl->insertClassFieldAce('foo', $sid, 1);
     $acl->insertObjectAce($sid, 1);
     $acl->insertObjectFieldAce('foo', $sid, 1);
     ++$id;
     if ($depth > 0) {
         $acl->setParentAcl($this->getAcl($depth - 1));
     }
     return $acl;
 }