public function & find($path, $create=false, callable $operation=null) {
        $create ? $value =& $this->fields : $value = $this->fields;
        foreach (explode('.',$path) as $tok) if ($create || isset($value[$tok])) {
          $value =& $value[$tok];
        } else {
          $value = $create ? $value :  null;  break;
        }
        if ( is_callable($operation) ) $operation($value);
        return $value;
    }