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