[Boost-users] [MPL] implementing a "trait"?

I want to mark certain types as being of a category of my invention (e.g. is_pretty), for purposes of using enable_if in the same manner is standard type traits such as is_arithmetic etc. The trait will be false by default, and I'd declare something to nominate types that should be seen to have that trait. I want it to follow inheritance; if B has been declared to be in my category, and C is derived from B, then C will also be in that category without needing to do anything more (though some way to turn it _off_ would be available). Can someone advise me on an easy way to structure this? Thanks, —John