As far as there being examples on the internet of people writing them, I've only ever seen fairly clumsy efforts that don't get finished, or round robin types. I've yet to see serious efforts, but then again, I haven't looked for a while.
I used to want to write one of those but I gave up the idea as too difficult. I admire anyone that can write one and do it well because I imagine the process of writing them must be tricky.