public function combine($another)
    {
        if (is_a($another, DC_DataHolder::getKlass())) {
            // debug_message("entering combine...");
            // debug_var_dump("another", $another);
            //identifier
            if (empty($this->identifier)) {
                debug_message("DC_DataHolder::identifier should not be empty!!");
                $this->identifier = $another->identifier;
            } elseif (strcmp(substr($this->identifier, - strlen($another->identifier)), $another->identifier) !== 0) {
                debug_var_dump('comparison', strcmp(substr($this->identifier, - strlen($another->identifier)), $another->identifier));
                error_log("found a different identifier for this element ".$this->identifier." != ".$another->identifier);
                return;
            }
            //continue with other elements
            $this->addElement('contributor', $another);
            $this->addElement('coverage', $another);
            $this->addElement('creator', $another);
            $this->addElement('date_', $another);
            $this->addElement('description', $another);
            $this->addElement('format_', $another);
            $this->addElement('language', $another);
            $this->addElement('publisher', $another);
            $this->addElement('relation', $another);
            $this->addElement('rights', $another);
            $this->addElement('source', $another);
            $this->addElement('subject', $another);
            $this->addElement('title', $another);
            $this->addElement('type_', $another);
        }
    }