Redirect after login with nevow guard

I'm using nevow.guard, and when someone logs in I would like to send
them to the URL they originally typed. It's possible to do this by
posting to the URL guard.LOGIN_AVATAR + other stuff, e.g. to go to
http://example.com/foo/bar after logging in, the login page should post
to http://example.com/__login__/foo/bar. This works nicely. The problem
is, the login page doesn't know that the user was trying to go to
foo/bar. How can it get that information? I would have thought that it
would be available as the segments argument in locateChild, but for some
reason it isn't. Before logging in, the segments argument is always
empty.

Peter.