public function isZombie()
    {
        $ref = $this->getReferences();

        if( $this->isPrimaryKey() and !$ref->isEmpty() ) {
            return true;
        }

        if( $this->notNull and !$ref->isEmpty() ) {
            return true;
        }

        // comment
        // if( \Bond\extract_tags( $this->get('comment') )
        return false;
    }