Supra (Latin for "above") is an academic and legal citation signal used when a writer desires to refer a reader to an earlier-cited authority. For example, an author wanting to refer to a source in his or her third footnote would cite: See supra note 3. Or for text in that note: See supra text accompanying note 3.\n\nBased on this passage, when do you use supra in legal writing?
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when a writer desires to refer a reader to an earlier - cited authority
The relevant sentence in the passage is: Supra (Latin for "above") is an academic and legal citation signal used when a writer desires to refer a reader to an earlier-cited authority.

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The answer: when a writer desires to refer a reader to an earlier - cited authority