self and virtual types

I've been listening on discussions in the lab lately on handling self types. Sam mentioned something called virtual types. I know nothing about them.

However, I happened to run into an article by Remy and Vouillon arguing that virtual types (or at least common uses of virtual types) do not need special treatment in O'Caml, and gives some example code.

So I want to go back over the article later and really digest what is going on. I followed the argument as presented when I read it directly, but I'm feeling I may have missed the point when they were talking about camels at the end.