Zorn's lemma, an Uncommon card

The card carries these facts from its Wikipedia article.

• It states that a partially ordered set containing upper bounds for every chain necessarily contains at least one maximal element.
• The lemma was proven by Kazimierz Kuratowski in 1922 and independently by Max Zorn in 1935.
• It occurs in the proofs of several theorems of crucial importance, for instance the Hahn–Banach theorem in functional analysis, the theorem that every vector space has a basis, Tychonoff's theorem in topology stating that every product of compact spaces is compact, and the theorems in abstract algebra that in a ring with identity every proper ideal is contained in a maximal ideal and that every…

Rarity in WikiCards follows a year of Wikipedia page views: the most read articles become Epic and Legendary cards. The Zorn's lemma article was read about 72,000 times in a year. ATK comes from the length of the article and DEF from its quality, so this card hits with 75 and blocks with 55: each hit it takes is cut by 22 %.

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The cards just above and just below Zorn's lemma in the power ranking, all domains together.