private function getMaskAtIndex(AclInterface $acl, $index)
    {
        $objectAces = $acl->getObjectAces();
        /* @var $ace AuditableEntryInterface */
        $ace = $objectAces[$index];
        $securityIdentity = $ace->getSecurityIdentity();
        if ($securityIdentity instanceof RoleSecurityIdentity) {
            return $ace->getMask();
        }

        return false;
    }