The Atom Syndication Format specification is silent on the matter of what the form that a term can take, so nothing stops you from creating a hierarchical structure of categories.
Atom Syndication Format规范对term可以采用何种形式并无要求，因此您完全可以自由地创建类别的层次结构。
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At last, discusses the important role of action refinement in specification language's hierarchical descriptions.