:: Ord a => [a] -> [a] -package:html-entities -package:lens -package:imports -package:base -package:ghc -package:extra -package:vector-algorithms -package:generic-deriving -is:exact -package:Cabal-syntax -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.