:: Ord a => [a] -> [a] -package:list-t -package:imports -package:base -package:Cabal -package:ghc -package:basement -package:Cabal-syntax -package:extra -is:exact -package:integration -is:exact -package:statistics package:parsers
Take a parser that may consume input, and on failure, go back to where
we started and fail as if we didn't consume input.