}, renderRequest, request.url)

        def renderRequest(url):
            renderHeader(url)
            renderBody(url)

        def renderHeader(url):
            return "the header"

        def renderBody(url):
            return "the body (request id=%r)" % (get("request-id"),)

    This should be used sparingly, since the lack of a clear connection between
    the two halves can result in code which is difficult to understand and
    maintain.

    @ivar contexts: A C{list} of C{dict}s tracking the context state.  Each new
        L{ContextTracker.callWithContext} pushes a new C{dict} onto this stack
        for the duration of the call, making the data available to the function
        called and restoring the previous data once it is complete..
    c