While mapping an author element to an author instance isn't too bad, there's not grouping element, like authors, that makes it clear that each author belongs to a single collection in a Book instance.
尽管把一个author元素映射到一个author实例没什么问题，但是没有分组元素，比如authors，它清楚地显示出所有作者属于Book实例中的一个集合。
This season he's scored against Arsenal, AC Milan and Liverpool - not a bad collection.
本赛季他对AC米兰、阿森纳和利物浦都有进球——不坏。
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