有读者问： 有这样一句话：
(1)Should the named vessel arrive earlier than the date of arrival as previously notified to the sellers, the buyer shall advise the sellers in time.
这里的shall意味着什么？
句中的shall，是常用在契约、章程等中表“必须”的一个情态动词，再如：
(2)The seller shall provide all necessary information for the maintenance of the machine.
(3)The vendor shall maintain the equipment in good repair.
