append :: (Monoid a, Argument arg, Singleton arg a) =>
           OptBuilding arg a
append = setter (\a acc -> return $ acc <> singleton a)
append :: (Monoid a, Argument arg, Singleton arg a) =>
           OptBuilding arg a
append = setter (\a acc -> return $ acc <> singleton a)
append = setter (\a acc -> return $ acc <> singleton a)