[Haskell-cafe] Return user state in Parsec

Hi, I'm currently writing a parser using the Parsec library. What I want is to store the order of each subparser called in a user state. So every single parser will be marked with a label that is stored in a special treelike structure when the parser is run. My problem is to return the last state of this structure when a parse error occurred. This information shall be used to display a kind of stack trace showing the order of the parser calls for debug purposes. Any ideas how to achieve this? Thanks, Maciej