1 files changed, 3 insertions, 3 deletions
diff --git a/src/Text/Pandoc/Shared.hs b/src/Text/Pandoc/Shared.hs
index aa07c81e1..b5efcf172 100644
--- a/src/Text/Pandoc/Shared.hs
+++ b/src/Text/Pandoc/Shared.hs
@@ -706,14 +706,14 @@ headerLtEq _ _ = False
 
 -- | Generate a unique identifier from a list of inlines.
 -- Second argument is a list of already used identifiers.
-uniqueIdent :: [Inline] -> [String] -> String
+uniqueIdent :: [Inline] -> Set.Set String -> String
 uniqueIdent title' usedIdents
   =  let baseIdent = case inlineListToIdentifier title' of
                         ""   -> "section"
                         x    -> x
          numIdent n = baseIdent ++ "-" ++ show n
-     in  if baseIdent `elem` usedIdents
-           then case find (\x -> numIdent x `notElem` usedIdents) ([1..60000] :: [Int]) of
+     in  if baseIdent `Set.member` usedIdents
+           then case find (\x -> not $ numIdent x `Set.member` usedIdents) ([1..60000] :: [Int]) of
                   Just x  -> numIdent x
                   Nothing -> baseIdent   -- if we have more than 60,000, allow repeats
            else baseIdent