hello, you might also want to take a look at the "new monad library" i am working on. it is based on the old one, but does some things slightly different. one of the differences is the "nondeterminism transformer" whose implementation is inbetween lists and continuations. it also resembles resumptions quite a bit. i came up with this particular implementation but i would not be surprised if people already know about it. it is also quite simillar to the "term implementation" of the backtracking transformer described by ralf heinze in his "deriving monad transformers" paper. i would also be interested in general comments on the library. it is in the haskell cvs repository in fptools/libraries/monads (you need to check out fpconfig frist). bye iavor