[Haskell-beginners] monad transformers

[Michael P Mossey]

In Martin Grabmuller's tutorial "Monad Transformers Step by Step", found here

http://user.cs.tu-berlin.de/~magr/pub/Transformers.pdf

he gives an example of composing ErrorT, StateT, ReaderT, and WriterT. Early in 
the paper, where he composes just ErrorT and ReaderT

     type Eval3 a = ReaderT Env (ErrorT String Identity) a

he uses 'ask' and 'throwError' in some example code. I notice that he doesn't 
have to lift throwError into the ErrorT monad. Why is this? Do I misunderstand 
something about monad transformers? Is it a convenience definition of throwError?

Same thing the rest of the paper. I don't see anywhere he lifts anything.

Thanks,
Mike