Parsing combinators

Modified for producing documentation

• type P a
  nil   :: a -> P a
  token :: (String->Maybe a) -> String -> P a
  (!)   :: P a -> P a -> P a
  (<@)  :: P (a->b) -> P a -> P b
  (:--) :: P a -> String -> P a
  
  parse :: P a -> [String] -> Either ErrorMessage a
  usage :: P a -> String

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