Stacking up state transformers

[Guest, Simon]

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Thanks to Andrew, I'm now backtracking my state correctly.

Now what I want to do is have two elements of state, an element that gets backtracked,
and an element that doesn't.

My monad now looks like this:

type NondetState bs ns a = StateT bs (NondetT (StateT ns Maybe)) a

where bs is my backtracked state, and ns my non-backtracked state.

I can still access my backtracked state using Control.Monad.State.{get,put}, but
I can't access my non-backtracked state.

How do I burrow through the stack of monad transformers to get and put the 'ns' state?

Supplementary question: What documentation should I be reading?

cheers,
Simon

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