The library PFP  is quite useful for stochastic analysis and simulation. I have extended it from the IO monad to abstract monads. I put it into a darcs repository and add a Setup.lhs for Cabal . Unfortunately, I do not know its legal state, so that I can only offer a patch  in the darcs  format.
Well, actually I do not know how to create a pure patch with darcs. I can only find ways to publish the whole repository in the documentation of darcs. Therefore, my patch  is a stripped down repository. The recipe to complete it is:
Now you have a regular darcs repository in import.
 http://home.arcor.de/s.ka/haskell/pfp/export_2006-07-17.tar.bz2 Signature: http://home.arcor.de/s.ka/haskell/pfp/export_2006-07-17.tar.bz2.asc Legal matters: Copyright (C) 2006 Stefan Karrmann, Only Rights given by the LGPL-2 are granted.
 http://home.arcor.de/s.ka/haskell/pfp/create-repo.sh Signature: http://home.arcor.de/s.ka/haskell/pfp/create-repo.sh.asc Legal matters: Copyright (C) 2006 Stefan Karrmann, Only Rights given by the LGPL-2 are granted.
Check it! Check it!