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Validation failures\n  accumulate, so a whole request can report every problem at once.\n\nCollections are also checked by size: ?+ (at least one), ?! (exactly\n  one), ?∅ (none), and ?∃ (at least one element succeeded; keeps the\n  successes).\n\nClassifiers: ?\u003e tags by a predicate. Any function into Either,\n  Maybe, Bool or Validation is a classifier, applied with \u003c\u0026\u003e.\n  Railroad.Bifurcate provides neg, atLeastOne, exactlyOne,\n  noneAtAll and anySucceeds.\n\nWorks in any MonadError (Railroad.MonadError: mtl, ExceptT,\nservant's Handler) and in effectful with the Error effect\n(Railroad). All operators are infixl 1 like \u003e\u003e= and \u003c\u0026\u003e, so\nthey chain without parentheses. 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