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Def.Universe/U-set/U-small set/U-category/U-small category
Def.fully faithful/essentially surjective
Def.essentially U-small
Def. half-full fucntor/half-full subcatrgory
Def.Yoneda functors
Yoneda lemma
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来源：https://www.cnblogs.com/zny14/p/12337494.html