code to convert userdata to table?

[mitch abaza <mitch.abaza@...>]

My Lua code is calling a third party C++ library that returns userdata.  I need to serialize said userdata into JSON, which seems like a tricky proposition. If I can transform the userdata object into a table, then I can leverage existing lua packages to get my data into JSON.  Can anyone provide generic code that can transform userdata into tables?

-Mitch