"Edward Diener"
When I first realized this, I was thinking about bumping up BOOST_MPL_LIMIT_METAFUNCTION_ARITY internally, so that, in fact, the maximum supported arity of placeholder expressions (but not everything else) would be BOOST_MPL_LIMIT_METAFUNCTION_ARITY + 1, but unfortunately never got to it. I'll consider fixing this for 1.33.
Thanks for your feedback,
Why not have the answers to the exercises in the book on-line so that readers can look at them to further learn the library ?
Someone has to write up the answers in a suitable form. That could amount to a lot of work, and we were about at our limit with finishing the book. People have volunteered to do that for other books. Maybe you'd like to take up the challenge? ;-) -- Dave Abrahams Boost Consulting www.boost-consulting.com