Bourbaki dangerous bend symbol
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The dangerous bend symbol was created by Bourbaki and appears in the margins of mathematics books written by the group. It is analogous with a street sign that indicates a "dangerous bend" in the road ahead. Bourbaki, and those who later adopted the symbol, would use it to mark passages that were tricky on a first reading or contained an especially difficult argument. For example, computer scientist Donald Knuth uses it in his textbooks.
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