在我们学校，从学校自助餐厅偷取餐具几乎成了一个大学生的必要条件，甚至还有人都已经收集了一套完整的瓷器。
As an undergraduate, at my school it was practically a requirement to steal silverware from the campus cafeteria. There were students who'd commandeered full sets of china.
科学家们在2010年1月采取步骤进一步接近这个目标，他们宣布他们已经建成了一个悬浮磁铁，创造了被认为是实现核聚变的一些必要条件。
Scientists took a step closer to this goal in January 2010 when they announced they'd built a levitating magnet that created some of the conditions thought to be necessary for fusion.
即，“购买购物车中的商品”用例必须要使用“注册用户”，但“注册用户”只是在“浏览商品目录”之前-- 它并不是一个必要条件。
That is, "registering a customer" is required by the "buying items in shopping cart" use case, but it simply precedes the "browsing the catalog" use case -- it's not a requirement.
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