function leaf($name, $default = null, $function = 'df_leaf') {
		/** @var string $key */
		$key = "$name::$function";
		if (!isset($this->{__METHOD__}[$key])) {
			$this->{__METHOD__}[$key] = df_n_set(call_user_func($function, $this->e()->{$name}));
		}
		/** @var string|null $result */
		$result = df_n_get($this->{__METHOD__}[$key]);
		return df_if1(is_null($result), $default, $result);
	}