对立四边形：修订间差异

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 == 外部連結 ==
-* [http://www.square-of-opposition.org/ International Congress on the Square of Opposition] {{Wayback|url=http://www.square-of-opposition.org/ |date=20210211064003 }}
+* [http://www.square-of-opposition.org/ International Congress on the Square of Opposition]
 * [http://www.springerlink.com/content/l8rj3631747w/  Special Issue of Logica Universalis Vol2 N1 (2008) on the Square of Opposition]{{Dead link|date=2020年2月 |bot=InternetArchiveBot |fix-attempted=yes }}
-* [http://plato.stanford.edu/entries/square/ Stanford Encyclopedia of Philosophy article] {{Wayback|url=http://plato.stanford.edu/entries/square/ |date=20201204220258 }}
+* [http://plato.stanford.edu/entries/square/ Stanford Encyclopedia of Philosophy article]
 * [https://web.archive.org/web/20060507164846/http://uk.geocities.com/frege%40btinternet.com/cantor/Eximport.htm History of the Problem of Existential Import]