emS :: Eq a => a -> Skel a -> Bool
elemS c0 = go
  where
    go = \case
      Zero         -> False
      Act c        -> c == c0
      Op _ sk0 sk1 -> go sk0 || go sk1


elemS :: Eq a => a -> Skel a -> Bool
elemS c0 = go
  where
    go = \case
      Zero         -> False
      Act c        -> c == c0
      Op _ sk0 sk1 -> go sk0 || go sk1
elemS c0 = go
  where
    go = \case
      Zero         -> False
      Act c        -> c == c0
      Op _ sk0 sk1 -> go sk0 || go sk1