public function diff( \PhpSets\Set $set ) {
        $entries = $this->entries();
        $iterator = $set->entries();
        $intersect = new \PhpSets\Set;

        // check $set values
        while ( $entries->valid() ) {
            if ( ! $set->has( $entries->current() ) ) {
                $intersect->add( $entries->current() );
            }

            $entries->next();
        }

        // check $additionalSet values
        while ( $iterator->valid() ) {
            if ( ! $this->has( $iterator->current() ) ) {
                $intersect->add( $iterator->current() );
            }

            $iterator->next();
        }

        return $intersect;
    }