public function fillRange(Scope $head,$start,$end)
    {
        
        $start_index  = Utf8::ord($start);
        $ending_index = Utf8::ord($end);
        
        if($ending_index < $start_index ) {
            throw new ParserException(sprintf('Character class range %s - %s is out of order',$start,$end));
        }
        
        for($i = $start_index; $i <= $ending_index; $i++) {
            $head->setLiteral($i,Utf8::chr($i));
        }    
    }