Subset is a mathematical concept. If any element of set A is an element of set B (any a∈A is a∈B), then set A is called a subset of set B. ..
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"Classics and History Subsets" are four categories of ancient books divided by the ancients according to their contents. Some large-scale ancient books series often include four books and are named after them, such as Four Ku Quanshu, Four Series and Four Essentials. It can be seen that the four categories are of great significance to ancient books. Classics: Classics refer to Confucian classics; History: historical books, that is, official history; Zi: Hundreds of works in the pre-Qin period, religion; Collection: Poetry. Generally refers to the ancient classics of China.