nOffs accumulate in reverse order, so we use foldr in order to
-- process them in the right order
flattenExtensionFlags :: Maybe Language -> [OnOff ExtensionFlag] -> IntSet
flattenExtensionFlags ml = foldr f defaultExtensionFlags
    where f (On f)  flags = IntSet.insert (fromEnum f) flags
          f (Off f) flags = IntSet.delete (fromEnum f) flags
          defaultExtensionFlags = IntSet.fromList (map fromEnum (languageExtensions ml))

la
flattenExtensionFlags :: Maybe Language -> [OnOff ExtensionFlag] -> IntSet
flattenExtensionFlags ml = foldr f defaultExtensionFlags
    where f (On f)  flags = IntSet.insert (fromEnum f) flags
          f (Off f) flags = IntSet.delete (fromEnum f) flags
          defaultExtensionFlags = IntSet.fromList (map fromEnum (languageExtensions ml))
flattenExtensionFlags ml = foldr f defaultExtensionFlags
    where f (On f)  flags = IntSet.insert (fromEnum f) flags
          f (Off f) flags = IntSet.delete (fromEnum f) flags
          defaultExtensionFlags = IntSet.fromList (map fromEnum (languageExtensions ml))