MPL?

Whenever I start feeling proud of myself due to my C++ prowess, I mosey over to the boost developer's list and am promptly humbled. Recently there was a discussion started by David Abrahams about how great MPL is. There were various comments that the following example was confusing or that it was perfectly clear, but nobody hinted toward what it actually does. I'd love to be excited about the current and upcoming possibilities in boost, but I just can't wrap my brain around this metaprogramming thing. Can anybody explain in English what the following does? Posted by David Abrahams on boost developers list:

Have I mentioned recently how totally cool MPL's lambda facility is? I can't believe I can do things like this, even with MSVC6. It makes my metaprograms SO much easier to manage! mpl::logical_and< is_reference_to_classmpl::_ > , mpl::logical_not< is_reference_to_argsmpl::_ >

Thanks, Jeff