Andrew Bromage wrote:

> > You may have meant to stack the monad transformers in a different
> > order.

I finally understand.  This was exactly my problem.  I had to discard my state monad,
and use a state monad transformer on the simple backtracking monad (not the 
backtracking monad transformer on the state monad).

Now it works fine (of course).  Thanks for the help.

cheers,
Simon
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