[Haskell-cafe] Using multiplate to get free variables from a syntax tree

Hi all, I'm reading the haskellwiki article on multiplate. Is it possible to modify the getVariablesPlate example to return the free variables? One idea I had is to store an environment in a reader monad, and use local to update the environment at a let expression. Couldn't figure it out, though. Any ideas? Thanks! -matt http://www.haskell.org/haskellwiki/Multiplate