unique :: SubseqExtractor v loc a -> ErrorExtractor v loc a
unique ex = extractor $ \note -> case runSubseq ex note of
  [Pure a      ] -> Just a
  [Ranged a _ _] -> Just a
  _              -> Nothing
unique :: SubseqExtractor v loc a -> ErrorExtractor v loc a
unique ex = extractor $ \note -> case runSubseq ex note of
  [Pure a      ] -> Just a
  [Ranged a _ _] -> Just a
  _              -> Nothing
unique ex = extractor $ \note -> case runSubseq ex note of
  [Pure a      ] -> Just a
  [Ranged a _ _] -> Just a
  _              -> Nothing