private function findNonExistingUserName()
    {
        $userManager = $this->getUserManager();
        for ( $i = 0; $i < 20; $i++ )
        {
            $username = substr(uniqid('User#', true), 0, 15);
            if ( !$userManager->checkUserExistenceByUsername( $username ) )
            {
                return $username;
            }
        }

        throw new \Exception( 'Possible endless loop when attempting to find a new name for User.' );
    }