[plt-scheme] Do servlets run in parallel threads?

I'm writing my web site in scheme using the web server, and I am
wondering whether or not I can rely on servlets being run
sequentially. I looked at the code, but I'm having trouble
understanding much of it since it makes use of many mzscheme features
I am not really familiar with.

I've written a servlet which tells me when it is run from two threads
at the same time (checking and setting a module variable), and as far
as I can tell it never happens. It would be nice to know for sure
though.

    Henk Boom