:: Ord a => [a] -> [a] -package:list-t -package:witherable -package:vector-algorithms -package:basement -package:Cabal-syntax -package:Cabal -package:ghc -package:generic-deriving -package:hxt -package:vector is:exact -package:base-prelude 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.