:: Ord a => [a] -> [a] -package:list-t -package:Cabal-syntax -package:Cabal -package:extra -package:imports -package:witherable -package:incipit-base -package:ghc -package:base-prelude -package:lens -package:base -package:vector-algorithms 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.