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In the lexicon of youth, which fate reserves For a bright manhood, there is no such word As “fail.”
— Edward George Earl Bulwer-Lytton · Richelieu

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Youth is imagined as having a vocabulary with failure left out. The promise of a bright adulthood makes that confidence sound like part of its destiny. Describing failure as an unknown word gives the passage its bold, almost absolute faith in possibility.
Context reviewed