:: Ord a => [a] -> [a] -package:hspec -package:witherable -package:basement -package:ghc -package:lens -package:Cabal -package:Cabal-syntax -package:hxt -is:exact -package:extra -package:vector -package:errors 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.