protected function _chars()
    {
        if (is_null($this->col_chars)) {
            $a = new A();
            $i = new N(0);

            while ($i->less($this->_length())) {
                $a->add(new C($this->sub($i->value)->value));
                $i->incr;
            }

            $this->col_chars = $a;
            unset($a);
            unset($i);
        }

        return $this->col_chars;
    }