:: Ord a => [a] -> [a] -package:html-entities -package:ghc -package:extra -package:incipit-base -package:lifted-base -package:Cabal -package:vector-algorithms -package:witherable -package:base-prelude -package:safe -package:statistics package:parsers -is:exact
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.