[boost] bool_set proposal (aka Boost.Tribool for standardization)

In the course of working on the proposal for standardization of Boost.Interval, we have produced a separate proposal named "bool_set: multi-valued logic", which is related to Boost.Tribool. (it is used as the return type of the comparison operators between intervals) This proposal is submitted to the mid-term mailing and will be available as N2046. The PDF can be found here for now : http://www-sop.inria.fr/geometrica/team/Sylvain.Pion/cxx/bool_set/bool_set_N... Any comment appreciated, especially from Boost.Tribool users. -- Herve, Guillaume, Sylvain