Graph.hs

    No instance for (MonadState
                       (Seq (Maybe [Int]), Set Int)
                       (StateT (Seq Int, Set Int) Data.Functor.Identity.Identity))
      arising from a use of `gets'
    In the second argument of `(<$>)', namely `gets fst'
    In a stmt of a 'do' block: (x :< xs) <- viewl <$> gets fst
    In the expression:
      do { (x :< xs) <- viewl <$> gets fst;
           modify $ const xs *** id;
           return x }