:: Ord a => [a] -> [a] -package:list-t -package:incipit-base -is:exact -package:statistics -package:Cabal-syntax -package:ghc -package:basement -package:extra -package:errors -package:witherable -package:vector is:exact 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.