:: Ord a => [a] -> [a] -package:html-entities -package:incipit-base -package:ghc -package:basement -package:safe -package:lifted-base -package:containers -package:hxt is:exact -package:witherable -package:lens -package:vector 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.