:: Ord a => [a] -> [a] -package:html-entities -package:witherable -package:incipit-base -package:extra is:exact -package:ghc -package:vector-algorithms -package:statistics -package:lens -package:basement -package:hxt -package:mono-traversable 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.