[boost] [TypeErasure] BOOST_TYPE_ERASURE_MEMBER

Hi Steven, all BOOST_TYPE_ERASURE_MEMBER is only mentioned in the reference, it could be useful to add it to the tutorial: I guess that most user concepts will start from member functions of a given type. A sample on how to specialize it for types not having the said member function would be also welcome. Finally, why did you choose not to include the member's signature in the macro ? Isn't this giving the way for hard to find mistakes ? Regards, Julien