Wildcards - the funky question marks where a type parameter should go - are a means of expressing a type constraint in terms of an unknown type.

通配符—使用一个奇怪的问号表示类型参数—是一种表示未知类型的类型约束的方法。

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Scientists have identified a previously unknown type of ancient human through analysis of DNA from a finger bone unearthed in a Siberian cave.

科学家们通过对一块西伯利亚某山洞出土的指骨进行分析，发现一种前所未知的古人种。

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Yes, type "U" refers to Unknown for the Nominal Annual Rate.

是的，“U”型指的是未知的名义增长率。

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While the exact reasons are unknown, a diet of foods that quickly release sugar into the bloodstream is associated with a greater risk of Type 2 diabetes.

VOA: standard.2010.06.25

To detect and explain this previously unknown type of gamma-ray burst is a real achievement for the University of Warwick.

BBC: NASA's Swift satellite observes massive supernova stars

In a disease where the cause and risk factors are unknown, this type of competition is probably the best way to find some basic insights.

FORBES: MPN Research Foundation Is Awarding $100K Concept Grants To Combat Myelofibrosis

The exact cause of Type 1 diabetes is unknown, but both genetics and environmental triggers play a role.

FORBES: Magazine Article