(=:) :: (MonadWriter [Opt t] (OptM t)) =>
        ((t -> IO t) -> a -> IO a)  -- ^ selector
     -> OptM t ()                   -- ^ options
     -> OptM a ()
(=:) f optm = tell . map (fmap f) $ runOptM optm
(=:) :: (MonadWriter [Opt t] (OptM t)) =>
        ((t -> IO t) -> a -> IO a)  -- ^ selector
     -> OptM t ()                   -- ^ options
     -> OptM a ()
(=:) f optm = tell . map (fmap f) $ runOptM optm
(=:) f optm = tell . map (fmap f) $ runOptM optm